Can NPCs go along for the ride?

Well I’m sure I can make a two seater bike, I simply want to know if it works before I waste a bunch of time getting what I need.

Tandem bikes only have one set of foot pedals, so it’s not a proper tandem bike. It looks like you can’t install a second even with 20 mechanics, too. The second person is merely extra weight – you’re better off with them riding on the back of a regular bike because at least the bike weighs less.