Interior Plumbing on geothermal heat pump

I have two geothermal heat pumps in my house, one for the first floor and one for the second. The water flow through the two heat pumps is in series. The water enters the first heat pump and moves to the second pump when it exits the first. When the water exits the second pump it splits and goes in parallel to two control valves controlled by each heat pump. That way if either pump is running the water flows through both. I have been advised to change to parallel so that each heat pump controls its own water flow independently. Does any one know if this is the best way. I have all of the plumbing parts to make the change tomorrow but I dont want to make a mistake.

Systems are more efficient if the temperature difference is high. By running it through one system then the other in series, the second one is getting warmed up water, and therefore working harder and less efficiently.