I dont see why not, you could OC or under clock them so they both run @ the same speed.
So how do you under clock them?
My second video card was an overclocked version. I ordered a regular version and it works fine. I just have them running at the same speed as the slowest one because only Crysis and S:CS are the only games that truly stress both GPUs. GTA4 is a beat, but it doesn't use SLI.
The fourth gpu in a 2x295 sli is virtually wasted - quad SLI doesn't scale well.
The second 295 is virtually wasted unless you are running at 2560. What's your screen res?