Can't find anything on this obvious problem.
Got 2 tracks which I recorded with right hand set of mics; and another 2 with the left hand set of mics (on a piano).   Merged the left and the right mics to one track each.   But now I want to turn these two tracks into one track with the right and left sides distinct, obviously, to preserve the stereo sound.  Seem to get one mono track when I pull them together (even though it has an upper and lower section and therefore looks like a stereo track.  The sound is definite mono.)

Thnks, guys.

I think the answer lies within what you did in Sonar which isn't stated in your post.
If it was me and I had 4 tracks of piano, I would have them all routed to a buss, and bounce the buss. (If I wanted to lessen the track count). Is that what you're after?

It is less destructive to leave them all as mono tracks and simply pan them to the desired place in the stereo field. I hope you saved your original 4 tracks. 
If this is to be part of a project with other instruments it gives you more option later on during mix down. 
If this is a solo piano piece then use the 4 tracks to make a stereo mix and export it. 

You don't have to hard pan the left and right mikes.