If I connected both ground (black) wires to one pole of the switch would flipping the switch cause both devices (breadboard arduino and laser) to turn on? Or is there something I don't know about the power that this would cause an issue?
it would work as you wish.
Current will flow through the circuit through the two power supplies and the Arduino if you simply lift the common ground.
It is simply NOT good practice even if it MIGHT work in SOME circumstances.