If I read correctly, there's something about some wiring not being capable of passing enough electrons to strike the bulbs. Considering you run all new wires in the first place, it seems to me that it makes more sense to make a beefier harness in the first place.
Anyway, the dual-in/dual-out harness turns out to be an extra 40A relay and power cables that insert between the HID harness and the ballasts. It means a second tap on the battery and a second wire run across the radiator. You *might* need it for 55W systems, but I'm going to go out on a limb and see if I can get away without them. It seems needlessly complicated to me.
Tell you what, if it turns out that you need them and I don't, then I'll give you mine. :-P