Timber, I needed similar to take my lad (12) or non cyclist wife on longer rides. I found a Dawes discovery frame on eBay – apparently the same as the double edge and build up using tougher parts from my spares collection.
After the frame, I bought some cheap but tough dirt jump sus forks as you don’t need much travel, FSA dh loose ball headset, and long cables with quality outers. Cross over drive came with the frame luckily.
All the other parts were discarded old parts, 36h downhill wheels ( been fine, keep them tight), Deore 9sp stuff, and 2.4″ tyres. Brakes are hydraulic discs, old clarks I think, but I updated the rotors to 203mm, and got a long hose for the rear.
It’s been great, and not missed a beat. Wheels have been strong enough, and brakes are fine also. Total spend was under £300 for frame chainset, forks, cables, headset and hose- it took a while to get the parts at those prices, but I guess it proves you can do it on a budget – it rides almost as good as my old racing tandem.
Good luck !