I just bought my first tandem two days ago, a new Cannondale Road Tandem 2. I am also in a decision mode about what, if anything, to use to indicate what gear we're in.
On my solo bike, I've ridden with a Flight Deck since it was new in 1996. I use it mainly for cadence, and telling me what gear I'm, quickly. With that FD it only takes a second, and probably less than that to read what gear I'm in. That keeps my eyes on the road, instead of looking at the cassette at the back of the bike (which is impossible to do on the tandem). I really like the virtual cadence also. No need for a cadence sensor, that's genius.
On the tandem, I've decided to install the Dura Ace in-line gear indicators on the shift cables at the handle bars, and see how that goes. I've never had a bike with those, and only seen a few in many years of bike riding that had them.
The Flight Deck is a great computer, one of the best in my book, but it lacks a feature I've long wanted in a bike computer. To be fair, most computers don't have it either though. That is a trip odometer that can be moved up or down to match the readings on a route sheet.
I have found only two computer makers that have that feature on some models, Sigma Sport and VDO. If the FD had that I'd go with it on the new tandem, too bad it doesn't.