As far as flow rate is concerned, you don't need two pumps. However, the real issue is really about heating the spa and not the waterfall so if you want to have a waterfall at the same time as the spa without sending heated water into the pool, then I would get a separate waterfall pump.
If you use 2" on the return, you really don't need 3" on the suction. 2 1/2" would suffice because the 2" will be the limiting factor so you won't notice any difference with the 3" vs 2 1/2" on the suction. As for the loop, if you can do a loop, that's fine but I would not use 1 1/2" pipe. The larger the pipe around the loop, the less a loop is even needed. Personally, I would use 2 1/2" suction and return pipe for the spa and loop that size around the spa. Smaller pipe results in more disparity between the jets, loop or not. The larger pipe removes the need for a loop because there is very little head loss between the jets.was advised to run 3" from drain and skimmer to pump and 2" return line to spa where it T's into a full circle of 1 1/2" to feed the jets.