when my crew does inclines with bands we set a pin in the hole that is like 6-8 inches off the ground, and double them up from there. turns out to be an arse load of tension at the top
From what I have read on Jumpstretch.com, the bands resistance is based on them stretched to 6ft. From the floor to the top of my lift, I don't think is more than 4.5 - 5ft.
Like I said though, if I ever need to double them, I will have to come up with a new idea.