Not to be contrary, but we (and a lot of folks on the lakes I frequent) pull two or even three behind our boat. Yes, the tubes will hit each other, its part of the fun. My kids a bit older now (teenagers), and if they go out with lots of friends multiple tubes is the rule. They love to have tube "wars" and jump from one to another.
You need to keep a sharp eye, as falling off is pretty much inevitable. But then again, skiers and boarders all eventually become swimmers, too.
With the right prop the little Merc can do it, but it would not be safe because the 2 tubes would undoubtedly hit each other. Not like skis where the skier has control. Tell your kids NO, you're in charge.