Hello there 
Much depends on how much food you expect to have to carry from the start. Setting that on one side for a moment...
These days, it really shouldn't be necessary for either of you to carry more than about 10-12kg baseweight (i.e. without food and water) whether your trip is going to last 3 days or 6 weeks, particularly bearing in mind that you can share the weight of things like tent and cooking equipment between you. Quite a lot of peeps these days would reduce that really quite significantly, but much depends upon what the two of you need to actually enjoy yourselves. I take things (camera, mp3 player, comfortable mattress, book(s) etc) that some peeps would leave behind, and it's in areas like that that we all need to decide how to make our personal compromise between weight and comfort.
Even so, though, with the lightweight kit now available it really should be possible for you to be carrying no more than 10-12kg baseweight and*still* taking things to make life enjoyable.
Food is heavy, though, so if you need to carry a lot then your weights are going to increase.
So, in order to help peeps give you more constructive assistance, can you say:
- how many days' food you expect to have to carry at any stage;
- what tent you plan to take;
- what rucksack, mattress, sleeping bag and stove you plan to carry?
Fwiw, I've been using a 50L pack (Osprey Atmos, because that one is comfortable for me) for a number of years now, whether I'm going out for 2 days or 3 weeks. I'd be taking it for any extended backpacking trip, because really the basic kit doesn't need to change a great deal (assuming a person plans to camp and cook).
It sounds like a wonderful trip! Can I come? *g*
p.s. you don't say how experienced you are at this sort of thing, and you might be looking for some pointers on what you might expect to have to carry. If that's the case then take a scan at the list of stuff I recently took (and left at home) when I walked for 14 days in Corsica. It's just a starting point, and others have similar lists. I found this sort of list very helpful when I first got back into this sort of walking, though. Welcome to backpacking
Corsican kit list here.