I would imagine that once you get to a certain diameter, the tube may be a little too big to have on one leg? And also, I bet that if a tube's diameter is small, it would be awkward to have connected to both legs. I have a tube and it is freakin' big, yo! I jumped it once head down (my 200th jump) and have only jumped it a handful of jumps since, all hand held... I have recently had the urge to bust it out and organize some tube jumps... but the weather is still crappy.
My tube is 20 inches in diameter and designed with a two leg attachment system. I haven't jumped the other system so I can't say what I would perfer to fly. For deployment I think a double attachment is best. My tube attaches in between my feet. I deploy with my legs straight out. With my legs straight the attachment system angles the tube 45 degrees to the ground. Air enters the tube and inflates it 6 inches behind my feet giving my canopy more clearance during opening.