If you're a more able cyclist carrying a partners kit is a great idea, it makes it more likely you'll travel as far/fast for the same relative effort.
I've done it a few times, with someone who'd only ridden more than twenty miles a couple of times, used a trailer and it worked well.
I carry most of our luggage on cycling trips or holidays involving cycling with my wife. It evens up our capabilities a bit.
Works well with my Raven Tour - front and rear Ortleib panniers with 72 litres capacity, plus a 30 litre rugged stuffsack on the rear rack top if mainly cycling, or a 48 litre rucksack if doing some walking as well. Wife has a pair of medium size panniers (about 35 litre capacity) on her 26" wheel bike. That is plenty of luggage space for a week of self-catering (but not camping). If I ever need to carry more, I can fit a second pair of rear panniers on the front instead of the smaller front panniers, and the rear rack will cope with the stuffsack and the rucksack if necessary.
A bit more difficult for trips with our Brompton folders, so I have to be more severe about restricting the amount of stuff packed. I have a large front bag with about 35 litres capacity, plus the 48 litre rucksack on the Brompton's rear rack, stabilised by strapping the top of the rucksack to the saddle rails. Wife has a lightly-loaded, slightly smaller front bag with about 30 litres capacity on her Brompton. With the two Bromptons folded and bagged, that means 5 items of luggage to load into the train or boat for multi-mode trips.
