My thought for a drop table is to make a 2×4 framed box like your design, but only attach the center 2×4 to the waste board and put a bottom on the box. Lift the waste and center board out to drop the work surface.
I read somewhere that when doing multiple nozzles, you need to specify the exact X/Y offset between the nozzles. I assume the Z has to be exactly the same or the lower one will hit the model when the higher one is printing.
So, I guess my only remaining question is about mounting two extruders. Does anyone have a design that uses the latest universal mount?