Same meter, then yes, you can, But --
A wise man will not use all the circuits in a new breaker box.
The day you find the one you forgot about, there is no place for it to go.
The prewired tail on the reliance manual boxes are just not designed to work the way you want.
Each circuit has to run from the wire that is under the breaker, to the reliance box and back to the breaker. So you will have to run more than a few wires between the panels.
It is easier to move 3 breakers from one box to the other than to try and wire 3 transfer circuits into each box.
It is wiser to spend the $250 - $300 and get another 6 circuit box. Then put one on each panel, and you will have spare circuits in each panel. You will find a use for some of them in the future.
It will also make it much less difficult to change to 50 amps of back-up, if/when you outgrow the 30 amps that only the one box will give you.
Ok you wrote a book. Proposing all manner of third rate hackery. And what does it boil down to? You want to get 5000W out of your 5000W generator. Quick question.
What is 240 x 21 ?
By my math, it's 5040. There's your 5000W. You do get it out of the big NEMA L14-20 connector.
I have no idea where you got 41A. I'm pretty sure you made that up, probably by dividing 5000 by 120. I seriously doubt it was on the generator spec. There's a way if you really really want that, but as you get educated, you will realize you do not.
What is it you're missing? The odd idiom of North American 2-pole service. I don't blame you for not getting it... It's weird.
Your house is served by +120V, neutral (0V), and -120V. I just described an instant in time, they're AC so they will reverse position 120 times a second. The poles are called L1 and L2 and the middle is Neutral.
240V loads grab L1 and L2. 120V loads grab either pole and neutral. Which pole they grab is nearly random and that's the idea, to make them average out so loads are balanced.
For you, with 21A on each pole, balancing is a big deal. You'll have a problem if you put 30A of load on one pole. So you'll need to get into the gory details of what is on which pole, and manage accordingly.
Step 1: Control MWBCs so they don't kill you
I don't recommend rearranging things on a panel because you can break a type of wiring called a multi-wire branch circuit. Find an electrician and tell him to do exactly this:
find every multi-wire branch circuit in my home, and make sure both its hot wires are served from the same 2-pole breaker.
Step 2: get rid of double-stuff breakers
If your panel is stuffed, and has lots of breakers that have 2 breakers in 1 space, those will drive you absolutely bat crazy. ack... You know what, to heck with all that.
Let's just get you a new subpanel with the appropriate interlocks, and move the loads you want the generator to power into this new subpanel. Make this subpanel quite large (at least 20 space) realizing you'll use 4 spaces just for the interlock.
In a perfect world, your new panel will have ammeters which will tell you how close to 21A each pole is getting. Even better get one of those new fangled whole house monitoring systems. Ask a new question on how to get one to work in a generator interlocked panel.
Step 3: rearrange your loads in the panel
Now finally, it's time to learn the gory details of how poles are assigned in a panel. Read my posting here. Your panel may differ, but probably not by much.
Move your loads into the new panel, and consciously and carefully balance the loads. For instance if your table saw is on L1, put your dust collector on L2. Stuff like that.
Best Answer
For a generator of that size, the primary connection would not be through those convenience outlets, but rather directly to lugs sized for the cables required. If this wasn't the primary use of the generator, and you wanted an easily detachable connection, and 50 amps was sufficient, then you could use the 4-conductor receptacle with a cable.
Your model sounds like the G50. See page 34 of that manual: