A power/ground distribution block is used to run one large power wire a location and then separate it to multiple wires to power several amps. Since you already have two power wires, there is no need to add more. I'm assuming one is for the sub amp and the other for the 4 channel amp. If you already have all those wires there and are not getting noise in the system, there is no need to change anything. Are you running the grounds all the way back to the battery?
The only wires to be concerned with running together are power and rca wires.