How many miles will each car be driven a day?
What size panel are you pulling from? What's its rating? What breaker spaces are available? What other draws happen at night?
Here are two options:
a) Find out the max current you can pull with 6gauge wire at the length you need (75'?) and put that size breaker in. Remember to use the 75Cdegree rating since that's probably what your connections will be rated at. Put two NEMA 14-50 outlets in series on this circuit. You may find a 60-amp breaker is legal/safe (do NOT take my word for it, I do not know).
b) Restrict the current on Roadster and Model S each to be 40% of the breaker you put in. That's because you're only supposed to run at 80% of the rating. Set each car to charge at plug-in time. You only need to do these settings once - the car will remember.
Now, both cars will charge as long as they need to.
The danger here is that if one car is driven a lot more than the other, it may not fully charge in time. Let's say you were able to install a 60-amp breaker. That means each car can charge at 24amps, or around 20 miles per hour. If you get home at 7pm and need to leave the next morning at 7am, you'll put up to 240 miles into each vehicle. That's a total full charge for Roadster. If you were only able to put a 50-amp breaker in, then you'd get about 16 miles per hour, or up to 192 miles. You can extend the math from there.
If you know one car was driven more than the other, you can vary the draws accordingly, as long as the total of the two cars doesn't exceed 80% of the breaker rating.
a) Put a subpanel in. That'll be a single run with bigger breaker and thicker wire. That'll cost more and be harder to pull.
b) Put in two NEMA 14-50 outlets, each on their own 50-amp breaker in the subpanel.
c) See #1 above for math. If you were able to put a 100amp line in, then you can let the cars run at their maximum UMC pull (best).
Before you go ahead, make sure you either get an electrician to do the work, or at least talk with an experienced electrician familiar with codes in your area. Get permits and get it inspected afterwards.
The really interesting future possibility is if both cars support something like OVMS. Then you can have the cars "talk" to each other, doing things like having the less essential car start its charge only after the other car has completed its charge.