Lift is roughly proportional to the square of the speed. So to get double the lift (if the weight doubled) you would need to go root 2 (or 1.414) times faster. The equation is:
http://en.wikipedia.org/wiki/Lift_%28force%29
where L = Lift
V = velocity
and for the rest see the wikipedia page.
PS: I can't see any reason why this doesn't hold say 1kt above stalling speed.