What length and how many straps would I...
What length and how many straps would I need to tie down two recreational kayaks on a kayak stacker?
In my experience its best to err on the side of having too much strap. I recommend a 2, 12 footers for your endeavor. You could possibly get away with 9's (depending on how wide your boat is)but when I secure boats with these I lay the strap over the boat and under whatever you're using as your "anchor" (roof rack, side rails, etc) then double back, so the extra length gives you something to pull with. I find this is the best approach to give you the most leverage...and consequently get your boat on the tightest.
Hopefully that helps.
I use two 12-15 foot straps for two whitewater boats. This should work well for two rec boats.