Gerard, you need to consider that the QDC connection adds additional resistance to the loop and will reduce effectiveness.
EK advise that if you're going with 2 or more GPUs to use a separate stronger pump than the default supplied in the Predator unit.
Obviously talking about the 360 unit here. You may also find that you would want to add in some additional radiator real estate as the 'standard' recommendation is 120mm per item with a spare 120mm. e.g. 1 x 360 will be fine for a CPU and GPU. You would find that the additional GPU would reduce overall system cooling effectiveness, but it's really the 2 sets of QDC that would cause the most concern off the standard pump.