Earlier quoted context omitted.
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…
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.
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.