When Marvin Gaye sang "It takes two baby" I doubt he was referring to a couple on a surfboard but if the International Tandem Surfing Association (yes ITSA mouthful) is looking for an anthem, it would surely suffice.
Like so much in surfing, the origins of tandem surfing owe much to the Hawaiians. The story goes that as tourists flocked to Waikiki at the beginning of the 20th Century to watch the likes of Duke Kahanmoku strut their stuff, the urge to "catch a wave" had the male visitors solo out and give it a go. Their wives and daughters, not to be outdone, hired the locals and were "escorted" out into the surf and shared a board.
I've watched Mr M at our club - one of those watermen with shockingly precocious wave riding skills - take his tandem board out with a lady from time to time. The stick is so big, watching him unsheath it from the board bag reminds me of Cold war footage of missiles emerging from silos. And his surf board's impressive too.
The body strength required of the male half of a tandem team usually means that stick insects need not apply even if the girl being carried is usually a slip of a thing. I read somewhere that there's no rule that says the girl can't carry the bloke. Now, there's something to draw a crowd. I haven't heard of same sex tandem teams but I'm sure it's only a matter of time.
The closest I've come to two on a board is when Spike (who's a she) rolled me onto a Rescue Board in an impromptu and rather impressive display of lifesaving skills. Of course, as evidenced below, it can all go a bit "Pete Tong" - which only highlights how skillful (if still slightly curious IMHO) the sport is.