Hm, switching right on the beat may be tricky, especially since not all music keeps its rhythm steady. Even if it does, you'll need a lot of precision just to know exactly when each beat starts. And setting an instant change to happen at a predetermined point in the future might not be feasible, though granted I haven't tried yet. I'll keep the idea on the backburner, but I can't make promises. If it's a SPECIFIC beat you want to switch on, there's a way to do it: write both into the same track, then loop only the first part. Then, change the loop once you want to "swap" tracks. But this will always swap in the exact same place every time.