Wheels do need to be tough, lightweight XC stuff will die quickly. Reliable hubs are a must too, we use a Hope Pro2 (20mm) on the front and a DT Swiss 540 (tandem rated) on the back. Forks will take a battering, but any decent freeridey type fork should be OK.
Definitely get a triple and don’t try and save weight on lightweight chains and cassettes, they won’t last.
Lizzie and I run Middleburn tandem chainsets on an otherwise XT drivetrain with decent cables (loooong cable runs) and a Deore 10 speed cassette and chain. Get a braided hose for the rear brake, much improves braking performance, with 200mm rotors. They will get very hot otherwise if you scrub speed off down a slope.
Think that’s about it, but any questions, do ask!