As long as you do not put the output in parallel you are OK. It is not the same as placing two 12 volt car batteries in parallel.
If the outputs are not Exactly equal, the PSU with the sligtly lower output becomes a load for the one with sligtly higher Eout.
Example - You can have PSU1 +12V go to PCIe of GPU 1 and PSU2 +12 V go to PCI-e of 2nd GPU card.
You do not want to put +12 for PSU1 to 1st 6 pin PCIe connector and +12 for PSU2 to the 2nd PCIe 6 pin connector on the same Graphics card (ie ATI X4870)
Myself, Just go with a Large Quality PSU. Find Max load Power and double it ( depending on +12 V rails may go 3X). Need a cheap watt meter from Newegg. May be surprised how low total Power is.