In my experience, mix recall takes a little while to load, so A/B this way has a delay.
If you just want to A/B two tracks in SONAR, solo one of the tracks, then right click the solo button, choose group, then A (or another group name and color). Then right click the other track's solo button (which should NOT be solo'd), and choose the same group. Now clicking either solo button reverses the solo of the two grouped solo buttons at the same time.
Cheers, Peter.