Well, actually they may run at the same time, unless they are set up to listen on the same port, in which case the one already running will just pop up.

FrostWire and LimeWire both don't allow multiple instances of them to be run at the same time (wouldn't make sense to do so anyway) and FrostWire/LimeWire cannot really distinguish between a FrostWire and a LimeWire client listening on the standard port...