There shouldn't be a problem with that if the cards are both based on Nvidia's reference design (the cooler doesn't count). Do you need all that performance though? The GX2 is pretty damn fast in my book already
One thing that has me concerned is clock/mem speeds. XFX (I have one too) has GX2s at many many speeds. Does it matter if you have a 640 vs 680 vs 700? You can't tell just by looking at them unless you pull up the exact model number and compare it on the XFX site.