You know, I see this sort of thing a lot. It really doesn't work well.
The power supply works by measuring the voltage at the output. If you have 2 of these with the outputs connected together, one is going to have a slightly lower threshold than the other one. This device is the one that's always going to be supplying juice while the other one with the higher threshold is essentially off.
This will continue until the lower threshold device overheats and shuts down.
Even if you split up the 5V wires between the power supplies, they're still connected on the motherboard. There may be enough resistance between them so that it works OK, but it's not guaranteed.