Yes there is. Download the script here
and then run it before you open xcode in each users account. It will open the simulator in such a way that for every user you can run a simulator. Also, running multiple instances of xcode and debugging on multiple devices under the same user seems to work fine for me at the moment (XCode 4, ios 4.3.3) however I still havent managed to find a way to open more than one simulator for the same user though.
