Trial trips would be great for figuring out what you really need and whether you are both in equal physical condition. You may find out what is necessary and what is not. Like rockchucker said, the variables are great, you'd need to tell us more to get better info. 70 miles over mountain passes and rugged terrain, is much different than 70 miles on the flat or easy rolling terrain. You can also use 2 60 liter packs and switch who carries the tent or the food . Unless you are both the same exact size, packs may be comfortable to carry or not. I'd rather carry less or more weight in a pack that fits me well. Actually, I'd rather carry lots of weight in a good pack than a little in a bad fitting one.