You don't say what PSI your pump operates at, but you have to consider the length of hose being used by borth operatives.
I'm not sure, but I think that if both hoses combined equal, say, 140m then it will be the equivalent of a single hose of the same length.
It will also depend on the diameter of the hose you use, the standard hose is fine for multiples of 50m, the 6ml stuff will suffer far greater flow loss, dropping to a bare trickle the longer the hose you use.
As 2 poles are in use, the maximum flowrate just has to be affected for the obvious reason that what was going through 1 pole is now going through 2 of them.
Combined with hose length I would imagine that this may be an explanation.
I am sure that should someone from either Ionics, Omnipole or a pump manufacturer read this then they will clarify properly.
I know of one guy who has a pump with 100 PSI output, and he frequently runs two poles without any problems whatsoever
Regards,
Ian