Currently (on my Mac at least), if you hold the Sync button and change the tempo of the other deck, the two will change together for just a second, and then stop following each other. This does not allow you to do an interesting BPM-change transition with two tracks playing at the same time.
One option would be to allow the user to hold the Sync button for as long as they want the two decks to stay at the same BPM.
Another option would be to allow a double-click or two-second hold to enable BPM matching with a tempo change. That way you wouldn’t have to hold the button and would have both hands free.
You could do it like Serato does. I forget exactly how they do it, but with their Smart Sync turned on, I think when you hit Sync, the Sync button stays lit, and it automatically matches that deck to the other deck? (someone check me on that)
In any case, it shouldn’t be hard to implement, and would greatly improve transitions! Thanks!