I believe you can though it may be a touch tedious...
If you set Sonar to be able to load more than one project (I believe that's the default but I disable that as a default here)...
Load both projects and copy/paste tracks from them into a new project (better safe than shoving one into the other)...
You may have to load any synths. I don't remember if they get copied with the associated MIDI tracks...
Keni