You can do it, I would not.
Your amp's positive and neg. cables should be the same size.
So, if you are running a 4 gauge + you should run a 4 gauge - cable too.
What you might want to do, is either run an other cable back to the battery or
run a distribution block between the battery and amps. Run both amps the to the distro block and then a larger wire to the battery.
You also, want to keep the ground wires as short as posible.