Starcraft – How to deal with 2v2 rushes

I play on a 2v2 team as Protoss, my ally is Terran.

1v1 rush handling is fairly straight-forward, and if you're prepared for it the defender gets the advantage because of quick reinforcements, high ground, etc.

In 2v2 however, when a rush comes, it's twice as strong, and the defender is at a disadvantage because:

• It's usually too early to join armies with your ally, so it's 2v1.
• As Protoss, when an early double rush comes in at 6 minutes, they are right at my production facilities. It's easy for one of them to deal with my units, and one of them to take out my pylons very quickly.
• There are more options for the opponent to see high ground: hallucinated colossus, overloards, or because the army is so big, just running straight up the ramp.

For example I just lost to a 6-minute marine+roach attack on Monlyth Ridge. We scouted and knew it was coming, but I don't know what we could have done to stop it. They killed my pylons while I tried to hold them off, thus I couldn't reinforce, thus they didn't even have to kill my base before going to kill my ally.

The typical answer to holding on in a 2v2 game when one ally is dying is: Die as slowly as possible. This is excellent later in the game, but against decent opponents at about 6-8 minutes, this isn't very powerful / possible because:

2. I have very few units to distract them with.
3. My map coverage & economy is not enough to start building dark templar or other surprise tech.
4. This early, they have a very clear idea of what options I have, so they are not afraid to just march into my base.
5. Luring them away from the base is too risky; without any defense in my base they will run in and kill production / economy.
6. Once they kill my production (which doesn't take long), they don't need to bother killing my buildings. They can march straight to the ally's base. They only need to keep a couple units in my base to prevent me from advancing.

My 2v2 team tends to win as long as we can hold off 6-8 minute rushes. Here are some ideas but I would like to hear your comments before I throw away lots of games experimenting:

• More sentries for forcefield. The advantage is this doesn't cost much of a compromise to my build as I can build them just as I build any other gateway unit. However, it only delays the inevitable and doesn't allow me to ramp up my production. It also prevents me from helping my ally if they go to him. So I am pretty scared to use FF in this scenario.
• Hallucination. They won't have detection at this point, and if they arrive at my front door and see 2 immortals and 6 stalkers, they may just run away. If not, it could absorb enough of their fire to buy us more time than a force field.
• Turtle. My ally and I prefer longer games so we just want to be able to hold off rushes. So should we do it decisively by turtling hard? Especially on Monlyth Ridge, could it be that doing a forge-first opening and quick-ish expand is the best way to fend off their early attacks? The idea would be to sacrifice early units, put all resources into defense and benefit from a healthier economy and play the game on our terms. But I am not sure static defense is strong enough for rushes like this anyway, and it's a lot of investment.

So, how does a 2v2 team defend against rushes like this as the different race options?

[Update, after accepting Sorean's answer]:

I want to say how I integrated Sorean's answer into my thinking and on the ladder. I think you can boil
the issues down to joining armies at your base means giving up defensive advantages we're used
to in 1v1. These advantages boil down mostly to positioning / base geography. Without that
issue, for example on Discord IV, the defense strategy is the same as 1v1.

So the response is that, if you want to compensate for the disadvantage,
then you'll need to emphasize more heavily the defensive capabilities of your race in a 2v2 defense. Fortunately Blizzard has given enough options to make up for the disadvantage in 2v2 (I think…).
Note that I really don't know much about Zerg or Terran, but I'll try to list this:

1. For Protoss:
   1. Sentries with force field – if you can't completely block the opponent, you can at least choke them into a more favorable position.
   2. Cannons, though usual cautions apply for these costly defenses. But in some defensive positions cannons can be favored over units for the amount of damage they deal, and they are useful later in the game to detect burrowed units / dark templar.
2. For Zerg:
   1. Static defense (but usual cautions still apply of course). Zerg can benefit even better because of being able to move static defense.
   2. Creep
   3. Queens with transfuse. Think spanishiwa and how he manages to defend with almost no units against a 4-gate.
3. For Terran:
   1. Bunkers
   2. Siege tanks. Because siege tanks can arrive earlier in the game than anything else with such a long range, they are great for defending main / natural bases.
   3. Supply depots. Not sure if this really deserves to go on this list, but the ability to raise / lower a supply depot can be so useful in making building positioning work in your favor, and out of the opponents' favor. We are starting to experiment, putting the 2nd supply depot in the protoss base if we're walling in.