I weigh just over 100 pounds and I have sat in both--and been pushed. The single's a bit of a tight fit for my behind, but that's another thread. DH says it wasn't difficult to push me at all, but I wasn't in it long, since it was just for laughs. I had no trouble pusing DD, about 70-75# at the time, last fall. They are great strollers, really easy to push and maneuver.
Have fun!