I thought this might be a fun/interesting topic to talk about since this forum is a little slow lately.
I've mostly doubled women in the past, but I have doubled a few guys before (not for long period of time though). Last weekend I doubled a guy from North Vancouver to Richmond on the back of my R1 (he's approx. 6'2")!!
He had his car parked at the shop where my bike was earlier in the day as I had borrowed it to pick up my bike. He and I ride around for the daytime (he on his own bike) and then when it got dark we dropped his bike off in North Van and had to go back to the shop to get his car. I am very nervous about being on the back of someone else's bike, so I said that he could ride on the back of mine and I'd drive him to get his car. We went through a roadblock, but the police didn't make us stop... I think they might have giggled though when they noticed that it was a girl with a long blonde ponytail out the back of my helmet and a guy on the back of my bike, pretty much towering over me.