usually when running two pumps you will want to make sure you fill up the reservoir when both pumps are full speed. because of the greater suction when using 2 pumps at full speed you should be able to add more liquid in the loop.
we've tested it with 2 pumps and just a tube (no water block or radiator) and we didn't have any problem.
Let me know if I can be of any assistance: (Call or email or forums).
Stephen
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