Hi CdnPolar. My husband and I do the same thing. We have travelled for 6 weeks, hand luggage only. Too bad if you see the same outfit 10 times in 6 weeks...one pair of hiking shoes wear on the plane and comfortable crocs, although not fashionable, for the ship.
If weight is the biggest issue, you could put heavier items in your knapsack, then once through "screening" while you check in and they tag your bag okay for cabin, when you get to gate area, do a swap around and repack. We have done this before with success.