Agree those Shimano STIs can be problematic to set up, but if you get them set up right they do work very nicely and are then trouble free. I think the issue compared with Campag is that with Shimano it either works or doesn't, whereas with Campag you can set it up so that it's nearly perfect and get away with it 99.9% of the time. On tandems its normally easier to get right than on single bikes as the chainstays are longer meaning the angle of the chain is smaller.

To get it right I follow the following procedure.
1) Set the height and angle of the front deraileur so that you can just get a small coin in between the big ring and the outer cage, i.e. 2mm. At that point the deraileur outer cage should be parallel with the big chainring.
2) Set the end stops of the deraileur so that the mech just shaves the chain in 52x12 and 30x32 or whatever your largest and smallest gears are.
3) Set the cable tension so that the shifter clicks as you shift up to the big ring; normally this will mean there's a tiny bit of slack in the cable on the small ring.
4) Check it all works; you may need to fine tune the adjustments, but remember if you change (1) you also need to reset (2) and (3). Key is to diagnose what's rubbing and think through what you should change to eliminate it. You should also check that after a shift is completed, the deraileur cage should no longer touch the chain. If it does, it will likely overshift at some point.
5) Once it works ensure all fixings are firm and then ONLY CHANGE THE CABLE TENSION while out on a ride.

Occasionally the thing still won't work and drops the chain regularly. If after many hours trying it still doesn't work a deda dog fang can be useful to catch the chain on downshifts or you can try slightly bend the lower half of the front deraileur cage to give more clearance. This was the only solution to get clean shifting on my old road bike.