Instruments automation trace only allows one target connection?
You could try installing OS X in a virtual machine, and having two instances of the virtual machine running simultaneously. I know that might not be what you are looking for, but it should work.
Try to connect all your devices to a Mac OS X server then try if that fails get the device to pass it to the other one and then pass to the mac
You cannot run UI Automation simultaneously on multiple devices. Interestingly, even Xcode 5 bots seem to have this same limitation. :-/