IMO I have no idea what I'm talking about. Going from a #6 to a #10, the #10 side will have less pressure than the #6 side. Where in the line is the fuel pressure gauge?

It seems like it would be easy enough, but I wouldn't trust 2 identical fuel pump systems to flow exactly alike, I would run one good one and carry a spare in the truck.

Propduster,
 If you want more volume throughout the whole system, which IMO you are trying to achieve then you need to start from the tanks/pick-ups and work your way forward keeping with the theme of larger lines. I just upgraded my pickups. You would be scratching your head on how small the pickup tubes are on these tanks. Remember they had to fee around 330HP-400HP in factory form. Also remember the more bends and "Y"s in a system the more resistance and this could cause volume drop. Make sure the tanks are vented as well. As you remove fuel you have to replace the fuel removed from the tank with air.
  T.J.

Im not trying to start anything but a Y is a lot less resrictive than a t. Also, after doing a little research I found that most stock, original, and after market gas caps are vented. I have the flip top style and checked them yesterday they are deffantly vented. Allthough I dont know if there are vented well enough for big hp motors. Im venting mine to be safe. Just orderd a set up from harden marine. It a nice set up.