1. they are already using sample / autotuned backing vocals, so the double vocal thing is easy - they can just back it / fake it the way they are doing with 'roxanne' etc.
2. they are already using click tracks (they use IEM's to facilitate this - look up IEM at wikipedia if yr not sure what they are) and there is speculation they even drop back into a click at the end of 'roxanne'... (notice how after that joyous seattle performance, the tempo is always the bloody same now at the start, and always, always at the end after the jam, sting plods back in with a slowed down dah dah daH DUN DUN DUN DUN DUN DUN on the bass before singing the chorus again? i suspect they are going back to a click for the backing vocal samples).
therefore, no logistical issues with synch I at all. only repetetive strain issues, if they stick with these same 20 songs for too long they will all get RSI.
i have been meaning to check all the autotune vocal tracks to see how many of them are being played to a click, a simple loop BPM check at start, middle and end would do it. i fear the worst, that the tempos are often locked in by click tracks