The answer to your question is "yes." Either you both need a 60 L, or one or both of you can use a smaller pack.

Like others said, gear items can vary greatly in weight and volume. I have two 2-person tents; one weighs about 4 and half pounds, the other barely 3 pounds. The lighter one is far less bulky than the other. The same goes for kitchen gear, sleeping pads, and especially sleeping bags. The volume also depends on how much and what type of clothing you're taking (do you feel the need to change clothes every day?) Share some of those details with us, and we can start talking about packs. If you don't have those details down, then you have some homework to do before your trip. (If you have questions as you do your homework, we can help with that, too. But you have to do the work to get far enough along to be able to ask the questions; there's no short cut.)