sometimes it's simpler. I re-routed soft panels when I had them so the water went in one corner and out the other, and ALL the return water passed through them. The efficiency of heating was amazing that way.
It's fundamental to thermodynamics that the faster the water flows through the panels without cavitating, the more heat is transferred. Usually, the limiting factors are a loss in pressure, or more pressure than the panels can tolerate before they start to fail and leak.

I successfully ran a 4'x10' (same as a 2x20) with a little tiny Intex cartridge pump this way, and was able to heat my parents' 13k AG with only 6 hours of direct sun a day. For both, I did it by re-routing the panels so water went in one corner and out the furthest.