I trip with my now-19 year old son each summer, usually 8-11 days.  We could single portage since Ben's quite strong (6'2", 240 pounds, offensive tackle).  He can easily carry 120+ pounds across a portage, but he doesn't like to.  We like some creature comforts and fish quite a bit, so we usually have enough stuff to double portage fairly easily.  Sometimes, one of us will double and the other single if one of us feels particularly strong at the moment, or just too darn lazy to double it...