I have 2 mono tracks from 2 mics on a piano and simply want to combine 2 mono tracks into 1 stereo track without converting, rendering in a way that could affect the sound quality.   Is this possible and how is it done?   I have Sonar X1 Producer Exanded..

Select both tracks, Edit-Bounce to Tracks and select Stereo as the Channel Format. 

Martin, forgive me if you already know this.
Your mono tracks are automatically centered in the stereo field. Simply bouncing them to a stereo track will result in a monophonic track that takes up twice the space on your hard drive. It will not give you stereo sound.
There must be a difference between the left and right. Stereo is essentially two mono tracks, and relies on that difference to create the illusion of space.
Personally, I wouldn't sweat the bounce. Pan one track 40-70% left, the other to the right an equal amount.
You may want to apply slightly different EQ to each, but that is a judgement call.
Unless you have a specific reason for a bounce at that point, I'd let it ride until the final export.
Just my thoughts. We all have different ways of doing this, so find a method that works for you.

I'd leave them as they are as well.

If you want to treat them as stereo when you mix, just route them to their own bus - call it "Stereo Piano" or something like that.

Leave the bus interleave at Stereo.

It isn't necessary to convert the piano tracks to stereo. In fact, leaving them mono has some advantages. However, bouncing them to a stereo track will not impair their quality in any way. 

invoke some channel tools as well for  stereo widening effects too.