Okay, so if I understood that right, the only real source of restriction you have in the system are the PA160's radiators?

In that instance, yes, pumps in parallel would be significantly better in very low restriction setups like the one you just described.

For a full system though, with a waterblock offering some measure of real restriction, things rapidly start to favor pumps in serial.

For example: Apogee GTX, PA120.2, 2 x Laing DDC with PTS tops

My apologies to the colour blind:



In all cases, pumps in series win. The reason is because of the waterblock. Notice how the series and parallel pump lines intersect at 11LPM (~3gpm). i.e. if your flow rates are above 3gpm, then having the pumps in parallel would be better (for that particular pair of pumps).