... actually it looks like you can ...
http://www.gpugrid.net/forum_thread.php?id=1505

Is this an attempt to get me to put my 285 onto the same mobo as the 295 and see me pie starved for a couple of days when I fail miserably

But really ... if you don't already have different cards to test this out with I will consider doing it. I think I may need to get a dummy plug