Wouldn't you need an additional step to mix the resulting stereo tracks down to a single stereo track? The M M stereo track is really just dual-mono. You'd want to mix the M/M stereo track with the -S/S stereo track. That way, after combining the two stereo tracks, you'd have a single stereo track remaining: one that's M + (-S) and one that's M + S.
Yes, that happens in the end when I "export as wav". It just says "I'm going to mix this down to a single stereo track OK?", and I let it.
I don't usually do a specific combine. After my above steps I generally spend time playing with the levels in an effort to achieve my desired mix/side mix/width, and getting levels I'm happy with. Then I export for tracking, and it's mixed down.