I have the same sort of setup with dual P7s (no external batt like yours however). works best if you use one on high and one on medium.
how waterproof are the cable leads coming out of the light?
I know mine were rain proof even in this weekend's downpour.
I agree, one on medium up close and one high aimed further is quite good. I run both on high when there are street lights so it will cut through. The leads seem quite waterproof since I sealed them with grommets but I'm going to keep an eye on it for sure.