If you're talking about a physical cash drawer, you could probably rig something up to get the same physical cash drawer attached to two POS terminals - they're pretty dumb, you're essentially just sending an electric pulse to open it. The easiest way to do it that doesn't involve butchering cables would probably be by using one driven by a printer, and then using an ESC/POS command through ISL to trigger. Or you could just splice the cables and make a Frankenstein cable.
Cash drawers aren't so expensive that I would ever consider doing this though... plus it will make a nightmare for cash management and throws out the ability to pin down who screwed the drawer up.