You can run two instances of the iOS simulator from the command line. They won’t be attached to Xcode debugging—indeed, it seems only to work if you do it without Xcode running at all.
First, you need to run the app in the simulator from Xcode, in order to get it installed in the simulator. Make sure you’re running the same simulators you’ll ultimately be using
Now open a Terminal window, and do this.
cd /Applications/Xcode.app/Contents/Developer/Applications open -n iOS\ Simulator.app open -n iOS\ Simulator.app
Update for Xcode 7: With Xcode 7 the application name of the simulator has changed, so it’s this instead:
cd /Applications/Xcode.app/Contents/Developer/Applications open -n Simulator.app open -n Simulator.app
When the second one launches you’ll get an error alert. Just dismiss it and select a different device from “Hardware” » “Device”. Now you have two simulators running, and whatever apps you already installed in them from Xcode will be there.