I tried putting my husband on the back. it was halarious to see. I'm 115 5'8" and he's 200 6'5". But we couldn't keep the front end down. we just popped up, light to light till we stopped laughing about it and we switched.
He has taken a friend of mine on the back for a 120 mile trip, she was probably a good 200 or so. he said the handling sucked and it wore down the back tire really fast. so I would say keep them under 150 and your good.