Bull
My guess is that a second inline pump would move more water, however the law of diminishing returns would kick in. If pump 1 produces 80% flow, adding pump 2 may only get you to 90%. The manufactures normally recommend a 20 deg temp drop through the exchanger, however in a perfect world the exchanger, heat flow, and water flow are all sized for each other. I think that more water flow would give more heat output with less water temp drop. In industry we often see devices or products that do not operate within theoretical values, but before commiting the time and money to solve the issue the question becomes "But is it still doing the job it was intended to do" If the exchanger in question is doing a satisfactory job of heating the space than it may be chasing a phantom problem. When I installed my system I used a lot of by-guess and by-gar engineering. It all seems to work. I don't think I should send a manned mission to Mars though.