These discussions are comparing apples and oranges though!
The key issue that is varying is engine hp.
The size of the section of pump that goes to high pressure is defined by the engine hp. One stage or two stage it does NOT matter! The small section is defined by the engine hp.
A two stage pump simply adds another section in the same body. When less than maximum psi is needed, the larger gearset adds into the flow. You get way more flow to use the entire engine hp.
BTW, I published in an earlier thread topic here I think, but a two stage pump runs the small one always to the outlet. The larger section either adds to the flow from the small one (say 3 + 8 = 11 gpm out) or it unloads back to the inlet at almost no pressure. Then the outlet is just the small one (3 gpm for instance). The pump does not vary its displacement in any way, just loads and unloads the larger section.
Sure, a single stage pump is constant speed, but the two stage is not 'slowing down' from that speed. The single stage pump STAYS on SLOW speed always, as the max size that the engine can pull. The two stage runs at the same speed in low, or speeds up when pressures are low.
My 18 hp Briggs flat twin runs a 28 gpm to 800 psi, then shifts to low for 7 gpm up to 2500 psi.
Or I could put on a single stage pump and run 7 gpm all day, and use under 10 hp most of the time. SLOW! Or I could put on a 28 gpm single stage pump and a 45 hp engine to turn it….
Do the math and make your own decisions.
Of all the splitters I have used, my main complaint is speed, it is very rarely low force.
The only place I would ever consider using a single stage pump is if there was a free, big engine already there running say a processor or elevator, or farm tractor PTO pump package. Then you have more than enough power, so no reason to use a two stage.
But for building a machine, why would anyone put on a 30 or 40 hp engine to run 11 gpm or so, fixed at this maximum, when an 18 hp can do 28 gpm most of the day, just slowing down to 7 gpm at the tough spots?