Some lakes in MN (Prior Lake where I live, for example) have local ordinances that require that the tow boat have enough capacity for all the passengers of the boat and the tube riders to legally ride in the boat. Also, in no-wake channels, the passengers have to all be in the boat - avoids kids falling off right in the middle of a bunch of traffic.
I would worry about the case when the tube submarines and turns into an instant sea anchor - you could easily overload the pylon and bend it. I have seen 4000 lb rated tube tow lines snap when the tube submarines (when being towed by a larger boat). If one rider falls off of one tube, its easy to forget about the other rider when turning around at low speed and have that tube submarine.
I agree with the other poster who mentioned that towing too much weight on a tube behind a PWC not nearly as fun for the riders or the driver. The lack of a larger wake, the inability to whip the tube out where you want it, and the constant fight to steer against the pull of the tube is much more challenging to make a fun ride than behind a larger boat.