SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I have a shallow bore pumping to two tanks in opposite directions; one through 90y of 2" polypipe and the other through 150y of 1 1/2" polypipe, with only about 5m of static head from the bore water level. I expect about twice as much flow though the 2" pipe which is okay, as it is a bigger tank. Problem is, I get only about half the TOTAL flow when pumping to both tanks as I get when pumping to either of them individually. I thought I might have a vacuum problem in the T-piece where the lines diverge, so changed the pump from the centre socket to one of the ends, which made no difference. As my 9-stage pump is probably operating below its optimum pressure range, I choked off the delivery by closing the bore head tap a bit, which only reduced the flow further. Spent yesterday cleaning both lines, but they weren't particularly dirty and it made no change. What to do, folks?