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.
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.
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.