You have to stand on two simultaneously.
I was half-joking, so I actually Googled it and guess maybe I was right.
http://boards.straightdope.com/sdmb/showthread.php?t=79112
“If I stand with my feet on two scales, what will their readings be?”
He’s so heavy when he stands on the scale it says, “One at a time.” - Rodney Dangerfield