[xenadu02]

You can't run multiple versions of Xcode concurrently due to CoreSimulator fighting over which version gets to run. This is a limitation we are aware of. We are also very aware of the problems it causes.

As for other problems, please file radars and respond to requests for additional information. I have been on the external side of radar, I know it can be frustrating, but we do read them and we do take direct action based on them. Even duplicates are very useful.

[megablast]

What? I have very little problem running Xcode 7 and Xcode 8 along side each other. As long as you close the simulator before running from the other version.

[xenadu02]

Only one CoreSimulator service can be running at a time because only one can be in control of the devices and the database. If you try to keep two versions of the tools open they'll keep killing each other's CoreSimulator or one will get stuck with the wrong incompatible version.

CoreSimulator is used during builds and for IB's accurate rendering feature. Instruments and Console open connections to CoreSimulator for various purposes. A lot of things can break.

[orbitur]

I've found having IB open in both versions will eventually lead to errors in IB, if not outright crashes.