Run several instances of the simulator and launch tests on them

As mentioned here it's possible to run two simulator instances using next command:

open -n /Applications/http://ift.tt/1OWD0fF --args -CurrentDeviceUDID $UDID

So using this command I can, for instance, launch simultaneously 'iPad 2' and 'iPad Retina' simulator instances. But when I try to run xcodebuild for testing (when I have already active simulator instances), it tries to use both simulators for testing inspite of destination option which I use in xcodebuild command. My purpose is to separate testing process between simulators.

Could someone tell is it possible to use "xcodebuild test" to test in specific simulator instance without touching another active instances?