I did something very similar last year, solo. In fact, this is my camping spot on the banks of Loch Lomond, the last night of the trip.
Here's some detail on my tarp setup, using the bike as a pole (6 nights out of 8, including the 3 on campsites; the other 2 nights had vegetation to attach the tarp to).
My luggage totalled about 35 litres, in the form of a 15 litre saddlebag, 7 litre handlebar bag, 7-8 litre frame bag (home made), plus a full length top tube bag (home made) and a couple of home made smaller bags. Bike tools and bottles were additional, mounted directly to the frame, including two home made bottle bags on the stem (one with a 3rd bottle, one with snacks).
This is what it looked like loaded (that's Loch Ness in the background)
(The home made bags are the pale grey and/or orange ones). Note that this is a very big frame and I'm tall, so there's a lot of room for luggage.
With 2 of you presumably sharing a tarp, cooking kit, and bike tools, you'll save a bit of space, but food, clothes, and sleeping bag/mat won't change compared to my setup.
I used a tarp, with another smaller one as a groundsheet, and carried minimal spare clothing. My sleeping mat was a very small light inflatable one. My stove was also minimal, only really suited to boiling water in an enamel mug, so I chose food to match (noodles, porridge, pasta).
Be sure to carry plenty of water if relying on dehydrated meals overnight, though boiling stream or even loch water for cooking should be ok. I carried most of my food for the first 4 days, but tried to buy proper meals when I could, at least once a day.
One thing I did was carry a small light backpack (Camelbak copy, without the bladder), strapped on the outside of my saddlebag where it could hold a little light stuff, but more importantly a few times I wore it from a shop to my camp or lunch spot, just a few km at a time. This allowed me to carry extra water for overnight, and fresh food to make a much more interesting dinner. It was also very useful for side trips on foot.
I've found a better photo, from a laden test ride, so there were a few changes: I carried more water for real (2nd cage, bag on stem), and I improved the strapping on of the backpack.
You mention having a tent at the moment. The difficulty with my tent, and part of the reason why I used a rack and panniers for last weekend's camping trip instead of my bikepacking luggage, is mainly the poles. They set a minimum length for the packed tent. The tarp is less bulky overall as well.
