First up a cheapo pair of shimano brakes from Merlin. I suspect they may arrive Euro-style, with front left and rear right. Is it as simple as loosening the connection, then swapping the hoses?
Could be but you may need a bleed if any air gets in and realistically the hoses will need shortening, a new barb and olive then probably a bleed
I’m also looking at the bigger 4pot Tektro Gemini brakes, to put on the tandem. This will likely require a new hose. How complex will it be to source the required parts and fit them?
Uberbike have a discount on their hose kits and I think they come with a single 3000mm piece of hose (check with them), I would get the appropriate kit from them if a longer hose than supplied with the brakes is required. Full bleed will be required.
All pretty straightforward but if you are asking then it may be worth giving to someone who has all the necessary kit and won’t mix up DOT and mineral etc