What is your rig?
Better to run off just one PSU rather than two and have more overhead than is useful.
Best way to do it, is to use a splitter which starts the PSU's up at the same time or it wont power up early enough to be detected on boot.
An example of one of these things is the '24pin Bitspower X-station'. You pay a little premium for the flashy-ness of it.