I've never even seen a 2 stage pump made to fit a tractor PTO. I had no idea they existed. For most applications, a 2 stage pump would not be desirable: Imagine your back working fast until it got under load, and only then slowing down - nudging the swing cylinder into precise position would be tough.
As
@CalG noted, you don't really need 2 stages if you buy a large enough pump. There are a number of 16-20 GPM pumps out there.
Most pumps seem to recommend a tank at least as big as the GPM rating. (Though my AmercianCLS log splitter has a 16 GPM pump, and I seriously doubt the tank is 16 gallons. I'll have to measure and calculate one of these days.)