I don't see there being stability issues besides those inherent with flashing a 6950.
If you only did the shader unlock though... well, 6970s operate with slightly lower VRAM timings and of course different clocks and voltage. The memory timings might cause some stuttering issues, and the clocks would basically go to waste unless you OC the 6950 to the same levels as well.
After a little research, it looks like the difference between 6950s in xfire and 6970s in xfire is not that great.
I am thinking that another 6950 may be the way to go here.
A lot of that would depend on the resolution you play at. At 1080p, you'll find you'll get CPU bottlenecked rather easily with crossfire, and you also go beyond your monitors refresh rate often, however at high resolutions there is a bigger difference. That said, it still isn't huge, so it may not be a bad idea to relock your 6950 and get another. It'll run quieter and cooler too.