We can easily run multiple instances of simulator using terminal commands. But they have be different devices like iPhone 6s, iPhone 6s Plus, iPhone 6 etc, i.e. we cannot have two simulators of iPhone 6 simultaneously. Also we can't use xcode debugging along with all the devices. We can only have a single simulator with xcode debugging.

Following is the terminal command to launch two simulators for xcode 7:

cd /Applications/Xcode.app/Contents/Developer/Applications
open -n Simulator.app
open -n Simulator.app

Every time we use the open -n Simulator.app with a simulator already running, it will launch a new simulator with error message "Unable to boot device in current state: Booted" Just click "OK" and then from hardware->device select any other device for the simulator and it will be launched!

After launching these simulator instances we can run from xcode with a new simulator and only on that simulator will xcode debugging work.

Also to run your application on a particular simulator, first the app must be installed from xcode on that simulator.