There is no restrcitions on how many PSU you can use with a single miner as long as it is more than 1 
Be careful with that. I've heard stories where ppl had that kind of configuration, and somehow turned one PS off while the other was still on.
The PS that was in the off state decided to short the +12V rail to ground, and Bad Things happened. The magic smoke got let out of some wires and traces and other things.