I actually had a lot of trouble finding an article or write up as to why NOT. The first search got me a whole bunch of stuff about lightbulbs for growin' drugs.
The second still didn't have anything useful over several pages, but I'd love some information here as to why not! I can only imagine problems if you mounted them both inside a case, and didn't consider the cooling needs.
EDIT:
Found a good write-up:
http://www.overclock.net/t/1177714/faq-dual-power-suppliesBut with little sources or technical reason besides obvious stuff.
What is made clear though, is it probably isn't that bad an idea if you're using it for more GPU rails, and I can't find anything contesting this.
It's a nice cost saving technique I'd love to be talked out of!