I would strongly recommend Campy 10 speed. 10 speed chains are plenty strong and durable enough for tandem use. I have Campy Centaur shifters and use a JTEK shiftmate so it works with a Shimano drivetrain. You can buy the Campy shifters pretty cheap from any number of UK online shops but I prefer Ribble.
Having two more cogs on the back will give more gears meaning either a wider range or smaller steps between gears. I prefer the ergonomics and shifting of Campy over Shimano and the shifters are rebuildable unlike Shimano which go into the trash when they wear out.