Straight up, this is a hard sell
First, EVSE's require a dedicated circuit. NEC 210.17.
Second, a 50A circuit can only serve one thing - NEC 210.23. So it cannot serve a hardwired driveway melter and also an EVSE receptacle. So that's right out.
Third, NEC 625.42 says the EVSE can be cord-and-plug connected if all are true:
1) it's part of a listed system
2) The receptacle <= 50A
3) It's cord connected to facilitate a) ready removal for interchange, b) maintenance and repair, or c) repositioning of the EVSE
4) Cord length <= 6 feet
5) Receptacles located to avoid physical damage to cord.
I think item 3 will be a hard sell.
So you're trying to argue that you should get an exception to the dedicated circuit rule (NEC 210.17); this outlet isn't an EVSE outlet -- it's a general purpose outlet that sometimes (wink, nudge) has a level 2 EVSE plugged into it. Hmmmm. That'll be an interesting sell to the AHJ. AHJ==your local inspector; the one that you pull the permit from.
The other prong of your argument, then, is that it's somehow reasonable for the driveway melter to be cord-and-plug-connected. In practice, that means it comes to a junction box, which then has a cord or inlet. This doesn't make a whole lot of sense, but it also absolutely requires the melter to call out a 50A breaker in its installation instructions. If the melter is looking for a 20A-40A breaker, no deal because the breakers don't match.
However, it would sell as a subpanel
If you convert this 50A circuit so it serves a subpanel instead of an EVSE, different deal. The subpanel does not need a "main breaker" if it's in the same building as it's served from.
In the subpanel you have a 50A breaker for the EVSE, and a ??A breaker for the driveway melter. That solves the breaker size problem.
Here's an interesting fact. Nothing in Code prohibits you from misusing a "generator interlock kit" to interlock any two random branch-circuit breakers, just for the chuckles. Only one can be on at a time. Go with either a Siemens/Murray panel or a Square D "QO". Use Siemens' ECSBPK02 interlock ($30), or a Square D's QO2DTI interlock ($25).
Now we open an ugly chapter: Provisioning. The feeder cable must be provisioned to be able to suppor--- Oh, wait. You've got an interlock there, don't you? Well, then. Nevermind.
The term you're looking for is EVSE, or Electric Vehicle Service equipment. You are correct; it is a relay, computer-controlled GFCI and a computer that talks to the charger on the car, including telling the car the current it is allowed to draw. This is a "soft setting" and is configured in the EVSE at commissioning time, either through DIP switches or in a special commissioning screen.
The proper charger inside the car listens to the EVSE data and charges at the rate authorized. If the car ignores this, the EVSE senses it and cuts power.
Therefore you are not under any obligation to have an EVSE be a particular size. You can simply determine the surplus ampacity available, and set the EVSE per Code requirements given that ampacity.
Start with a Load Calculation
using the NEC approved procedure for doing those. That is a science-based formula that determines probable loads for a given dwelling.
You do two Load Calculations, actually. One for the house's entire service, and the other for the loads inside the subpanel. Once you have finished that, you know how many amps of "headroom" you have to give to an EVSE.
For instance if you calculate to 144A on a 200A service, you have 56A of spare service amps. If you calculate to 24A on a 65A subpanel, you have 41A spare subpanel amps. This would call for a 40A EV charging circuit off the subpanel.
Provision the breaker and wires on this basis.
So continuing the 40A example, you wire the circuit with a 40A breaker and 40A wires (that being #8 copper or #6 aluminum).
At these large sizes there is nothing wrong with aluminum. There was an issue with 1970s small branch circuit wiring, but that isn't applicable here.
Then, derate the EVSE 125% / 80%
Any EVSE circuit requires a 125% derate, NEC 625.14. So you take the circuit size and multiply by 80% (the inverse of 125%). For instance, if you have a 40A EV charging circuit, you take 80% of it or 32 amps. This will be the actual charging rate.
Note that 32A x 125% = 40A.
They're not "singling out" EVs. This 125% / 80% thing is a requirement on any continuous load. NEC also imposes this on many other kinds of appliances that are arguably not continuous, to silence such arguments.
Now, this is configured into the EVSE. The EVSE manual will have a procedure for setting the maximum charge rate allowed; follow the procedure (a NEC 110.3 requirement). For instance, Tesla EVSEs have you set the breaker size (40A) not the actual-charge-rate (32A) and the EVSE figures out the 80% thing on its own.
The gory details: so much more than a relay.
If you want to be picayune, the EVSE doesn't actually limit current. The EVSE sends a signal (a square wave on the Control Pilot pin) which tells the EV how much it is allowed to draw. (32A in our running example). The EV onboard charger detects the signal and chooses to limit the current draw to less than that.
So the EVSE just passes on the message to the car, but the EVSE is the only place the charge rate can be set.
That is on purpose, as UL will not approve any setup where the consumer could change the max charge rate in software. It has to be "crack open the EVSE and change DIP switches" or some equally elaborate procedure.
It also means any car can be plugged into any EVSE and the right thing just happens.
If you have a limited amp allocation you want to dynamically share between two EVSE's, they have tech for that called Share2 that also works slick.
Best Answer
Honestly I would solve that by positioning the EVSE. If there are 2 separate garage doors, "between the doors" is a favored location. Otherwise midway down the side of the garage, back of the door mechanism so the far car can be charged by draping the cord over the nearer car. They would have to unhook the EVSE to leave. Alternately you could screw bicycle hooks to the ceiling joists and flick the EV cord up onto the bicycle hooks to clear the other car. Now the EVSE is close enough to the garage door.
But let's step back and take a bigger picture view.
Laying down the basic facts.
People starting into EVs tend to be pre-programmed with huge misunderstandings about how home charging works. These misunderstandings lead to bad decisions. Some of it comes from experience with other devices, or social media, or social media, or our #1 enemy: every car comes with a free TRAVEL EVSE for opportunity charging on the road, and comes with the plugs found on the road such as the sockets at RV parks for RVs. The thought was (mind you, this was 2011 and there was no charging network) that you could plan travel around RV parks. That's silly now.
So if you want to provision a NEMA 14-50 for guests who might have an RV or EV... go for it. Your idea of a mutually excluding interlock isn't dumb at all, and could be done using "side by side" generator interlocks such as ECSBPK02, QO2DTI or Eaton CHML. No need for silicon tech.
However, we can do better than that. Let's continue with the education, and let me just bang high points so you can skip what you already know.
Your interlock idea will work, but...
The problem is you are putting a NEMA 14-50 socket on one side of the interlock. And that means you need to provision 50A for that socket in your NEC Article 220 Load Calculation. And that means most likely you'll be getting a service upgrade which is likely to be at least a couple grand - more if it pushes you above 200A service.
But if it does work out, you can simply establish a subpanel with a $30 sliding-plate interlock between two breakers, such as Siemens ECSPBK02, Eaton CHML or Square D QO2DTI. Choose the panel make that fits the interlock - not every manufacturer has a $30 interlock.
But here's the other gotcha: GFCI. If you pull your permit after your state adopts NEC 2020, then you will probably need a $150 GFCI breaker on the RV outlet.
Hardwired EVSEs do not need GFCI protection since they are themselves GFCI receptacles. Think about it. Travel units can't be hardwired, but almost any wall-unit can. This little fact has a big impact on total price since by the time you buy a 50A GFCI and a QUALITY socket, you're most of the way to the cost of a wall unit, which needs neither.
Don't be skittish. Take advantage of Power Sharing tech.
They're way past that. Two matching EVSEs can talk to each other and coordinate sharing of a single allocation of power. They need to be hard-wired.
...And I gather the root problem is you're still "tire kicking" EVs and want to retain the option to angrily return the car to the lessor and tell them to keep it. So you don't want to sink a lot of cost in house-side infrstructure? Because we get a lot of that.
The answer is that yellow Romex that Alec mentioned at 32:55. Run a common 20A circuit to the garage - you'll use that for lots of things. For now, punch it down to a 240V breaker and use a NEMA 6-20 socket in the standard box. Order a $40 NEMA 6-20 dongle for that free travel EV and the dongle tells the car "20A circuit" (240V implied). You'll charge 140 miles in 14 hours and that's enough for getting comfortable with EVs.
Once you are comfortable, I would advise pulling the trigger on one EVSE which is equipped with Power Sharing technology - Tesla Wall Connector, CrippleCreek HCS, etc. Later if you see the need, you can buy the second one and implement Power Sharing between them. Maybe you can make do with one - if switching cars proves burdensome then get the second one and implement Power Sharing.
Note also that if you buy EVSEs that allow you to program the circuit ampacity... you can simply continue to use the 20A circuit you already wired. Just remove the socket and extend it to a suitable location for the EVSE. Then, build up experience charging two EVs on 20A. For most people, this will work. If it doesn't for you and having to top up at DC fast chargers become burdensome, then simply run a larger circuit and just reprogram the EVSEs. (Unfortunately CrippleCreek and a few others do not provide a way to reprogram, so avoid those).
Position your EVSEs with a thought for guests
Your default idea was to have one EVSE at the back of the garage, positioned to charge either car. A good start, but...
I would be choosy about positioning of two EVSEs (or even one) to allow for the possibility of guest charging, i.e. by running the cord outside. Obviously that favors positioning the EVSE near the garage door or near a side/back door depending on layout. We already covered the part where all EVSEs use the exact same connector except for shape, and cheap adapters take care of that.
As such, if your visitor knows you have an EV, **it is likely they expect to use your EVSE and wouldn't even use a 50A outlet. A lot of people don't even realize those things are travel EVSEs and simply leave them plugged into the wall at home and it would not occur to them to bring it. DC fast charging and 60A-class destination charging is now highly prolific, and there's no real need to limp into a KOA Kampground anymore to charge.
Installing an RV outlet for convenience of guests with RVs or travel EVSEs should only be done if a) your house can support the ampacity, b) you are willing to comply with state GFCI requirements, and c) you expect their guests to bring RVs or their travel EVSEs.