Agreed. Daisy chaining your shop vacs will not provide more power... rather more volume of pull at a quicker rate. In other words, you'll probably be able to pull the amount of volume of air out faster but it's not a stronger pull. Doesn't mean it won't work but it won't be as strong. What you're wanting is a higher pull of inches of mercury (in. Hg. vac) provided by a vac pump evacuating a tank(s) connected to your platen (which would likely vary depending on your elevation).
Go check out the TK560 forums and the vacuum forming section. All KINDS of good stuff to learn in there. Those guys know their stuff!! Hope that helps