If your considerations are in using a secondary power supply for the purpose of driving some coin door and track ball lighting in addition to having a PS that is driving the other PC components, I would suggest its really not necessary.
Unless you are using a particularily low wattage unit as your primary power source, you should have sufficient power in this primary PS to drive the remaining 12v and 5v light sources you are considering using the secondary unit for.
Edit: Didn't see the speaker amp component until a re-read, that might take a bit more in terms of amperage required.