My wife and I have been on a tandem for about 4 weeks. I would say "welcome to the club". When I initially got some cable stretch I had a devil of a time with the cassette, but if you're indexed a properly adjusted RD should not be a problem. Very hard for the stoker in any event to see the RD.
My FD is not indexed (it's a quad) and I'm still working on getting a feel with the Campy Ergo shifters. Work with your stoker in terms of trimming and I think the feel will develop. Go out on the bike by yourself on a quiet road and "look between your legs" (but don't spend much time there!). I've asked my stoker whenever I shift the FD to call out the chainring number which does help.