Unless you are renting gear, I recommend you each be self-contained. This will give you both valuable experience, and allow you each to plan your own meals. I suppose you could use one tent if you want to sleep in the same tent, and you could shift some things to account for that.

60 liters is likely too big. 50 liters should do it.

As a guess, you ought to be able to carry 22 pounds including the water plus 1.5 pounds of food a day. This will get you in the ballpark. This is without going ultra-light and keeping it simple.