I've seen a number of trailers like Timmac has, most were a put the boat on first, or vehicle below needs to be removed to get boat off the trailer. The boat is raised either manually with a winch, or a hydraulic setup is used to raise the upper item, usually a boat!
Like ANYTHING you buy that carries things. Make sure you have the axles setup to carry the weight of the items plus what part of the trailer they need to carry. I have seen MANY trailers like this, where the what you can carry is equal to the sum of the axels, "ASSUMING you carry enough hitch wt, ie the wt of the trailer!" so do the math carefully when buying and designing something like this!
Marty
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
2014 Chevy 1500 Dual cab 4x4
92 Red-e-haul 12K equipment trailer