Hey Kiterdan,
I have towed many a surfer, and other kiters without their kite ie on a wake board or twinny.
I have usually used the old suicide leash which is about 1.2m long. The surfers I towed across the channel between Lancelin Island and the North point which tends to be a big paddle and a bit sharky so they were stoked for the lift. Start with them laying on the board.
Also towed a mate on his twin tip, provided you are well powered the only hard bit is getting both of you out of the water, once planing it is really easy. Make sure your buddy doesn'y edge hard upwind of you or he will pull you off the water. Gybing is difficult unless you downturn the kite and keep lots of power throughout the turn otherwise you lose planing speed, sink and then power up to be lifted off the water.
It's fun! I've not towed into large waves, but it is only an extension of riding behind a kiter anyways so go for it, ya mates will be stoked!
Good winds,