Unless there is something very odd to these panels there should be two connections to each one and it really should not matter which is used for inlet and which for outlet -- it is a very simple heat exchanger. You have two of these, so you have a choice of using them in series or in parallel. If you use it in series (cold water in to unit one and hot water coming out goes to input of second unit) you will get the hottest possible water coming out but lower flow due to pressure drop. It is also possible that the heat of the outlet may exceed the temperature limits of the plastic -- you need to check that. If you run them in parallel you will get maximum flow, and get the most heat out of the system; althought the outlet temperature will not be as high as had you run them in series.