If you don’t want to have 7 boxes, I strongly recommend not to try a second 555PS.
Depending on what you are looking for, or what you like, adding a second power supply can make a massive difference.
I no longer have the ND555. But I did own a full 500DR series system (7 boxes) and the complete set of Super Lumina cables.
Here is my very subjective opinion regarding the addition of a second power supply :