I think you should be fine as long as you keep the 12v lines fat enough to deliver 10-20amp each.
The 2 12v input are to separate the bed/heater and "system" supplies as far as I know.
You'll need 800 watts minimum I think, but you should check the power supply lines maximum amp..
I use Corsair power supply for my PC and I think they can output 35amp per 12v rail, plenty enough for your use.
You could use the GPU connector since they deliver the bulk of the power.
I'd add some power switch on each 12v lines unless you want always run both printer than you turn on the PSU.