So while there is a slight bit more work to do with a second dinghy (shortening line, lengthening line, moving from one pontoon to another, keeping away from hot grill, etc.) I personally believe that 2 dinghies for that large of a group is not a bad idea.

We have towed dinghies of varying sizes in the past usually with a crew of 8 or more and have generally been pleased.

The two dinghies will offer you much more flexibility moving crew around, and certainly offer a more comfortable ride to/from shore, especially if you are planning to have dinner ashore, or eliminate a second trip.

Also, with 2 dinghies, no one must wait around for a ride.

I say do it, and ask what the recommended max towing speed is...