Thanks for the image Martin.

However, it doesn't make clear whether or not if, in series, the first pump will pump into the second pump's reservoir chamber or directly into the second pump inlet, though it appears it might be the former. The reason I ask is because it would then require that the second reservoir chamber is completely full (ie. no air) to ensure maximum transference of pressure / flow gain. Otherwise, it would be like one pump pumping into the reservoir that leads to a second pump - I may be wrong, but won't you lose some of the pressure gain in doing this?

Would it not be better by going in series and have one pump pump directly into the inlet of the second, rather than the chamber that feeds it? Isn't this the main reason why the XSPC res top does better than other res tops - because the pump pumps into its own inlet via that tube down the middle?