Another idea coming to us from California: Steve Boehne, the well know shaper of Dana Point Infinity Surboards, just finished his Quadnundrum, a board made for 4 paddlers.
The first purpose of this over 21 ft... craft is certainly racing, where the board has the advantage of 8 arms under the direction of a "captain", a bit like in rowing.
A little more on the reasonnable side for most of us: Steve also has some 16 ft tandem models and other ones for 3 paddlers, about 19ft long.
Whoever tried long distance or racing all know that there are some lonely moments, and these SUP projects seem very realistic... Will it become a new race category soon? To be followed...!