Supposedly that's more or less how Crysis 2 works, they only render 1 frame,
so it's not true stereoscopic, and then they interpolate the 2nd frame,
using depth values + sometimes projections are needed (more calculations),
but they don't cost so much overhead, so it's still a lot faster than rendering
twice!

It's obviously an approximation, but if the illusion works and is fast, then I take
it over a 100% stereoscopic solution, after all, all the best looking games are
based on illusion (2d trees etc).