Out of the Inyo pumps, the Super II is probably the best choice albeit an expensive one. You are getting close to the cost of a VS.
That's kind of what I was thinking and why the single pump option came to mind. Then I've also got the addition valve costs needed and not sure which would end up costing more and actually work better.
Hayward Super 2 Pump - Single, Dual or EE Pumps
www.inyopools.com
As for plumbing for a single pump, it is possible but you have to take a few precautions. Here is what I would suggest if you want to go this route.
1) Choose a high head, high HP VS pump such as the Intelliflo.
2) Combine both suction lines from the spa into a single 2 1/2" line to the pump.
Makes sense.
3a) After the pump, plumb a 3-way valve to select or partially select the spa jets or normal path through the filter/heater. This valve would be setup to direct most of the flow to the spa jets and part of the flow, to the heat pump and filter.
Makes sense. I had drawn out a filter bypass but a heater bypass with a control valve to limit flow through it to be just enough to get the heater flow switch makes sense. Minimizing head loss.
Optionally
3b) Have all the flow go through the filter and pump a bypass on the heater only. For this method, you want the lowest head loss filter possible which is a large cartridge filter.
That was the plan either way just for minimizing the amount of cleaning I have to do but also makes sense for this reason as well.
Without knowing for sure what is underground in the plumbing, it is hard to say which is going to work better. The problem with 3a is that you may need to redirect all the flow to the spa jets to get them to work properly and there may not be enough for the heater at the same time. The same issue with 3b except since you are recombining at the output of the heater, that may be enough for both.
But as you can see, you must have at least a partial bypass around the heater, that is the worst in terms of head loss.
