In addition to what rockchucher22 said, it'll make some sense for you and your friend to take a couple short trips, 1 or two nights each before the 70 mile hike. That'll give you a sense of who carries what, the size bags you need, etc., and after a few of these shorter trips you'll be able to create a system that'll help you become better prepared for the big hike and make it easier on you both.

Jumping into a 70 mile hike can be pretty daunting without a few trial runs behind you.