TandemGeek

First of all, how long has your stoker been cycling and how long have you had the tandem? If she is a cyclist, what type of shoe / pedal system is she comfortable with? My general advise to new tandem teams where the Captain is an experienced cyclist but the stoker is not is to "work up to" clipless pedals by starting off with quill pedals to deal with the various "fear factor" associated with clip-in pedals. It doesn't take long for most stokers to get a feel for the tandem and gain some confidence in their captain's ability to handle the thing which then makes the transition to a clipped-in foot a far sight easier. If she is already riding with some type of clipless pedal system, that's obviously what you'd want her to use on the tandem. One final thing to consider is why she might be worried about falling over... are you mindful that you've got someone sitting on the tandem with you and being careful to NOT LEAN the tandem to your down foot side when starting and stopping? The latter is something that many if not most single road bike riders do out of habit and if you do it on a tandem it can really freak out the stoker. I've never broken my habit 100% but Debbie has gotten confortable with it.

Back to the question at hand, IMHO MTB clipless pedals and shoes are easier to use than road pedals and are ideally suited for tandeming, particularly if you intend to do any social riding or tandem rallies. The latter is because MTB pedals/shoes allow you to walk around without being concerned about your footing or risking damage to your cleats / loosing a Kool Kover. However, roadies being roadies, you'll still see a lot of road bike shoes and pedals being used by the hard core folks who find MTB shoes and pedals just don't cut it for them on road bikes.

That said, IMHO, the easiest pedal to get into and out of are Speedplay's mountain biking pedal, Frogs. No springs, two-sided, and also easy on the knees. The various other MTB pedals from Shimano, Crank Bros, etc that use the SPD cleat come in a close second, again noting that they are multi-sided and relatively easy to twist out of.

In the road bike category, Speedplay's X-series pedals are perhaps the easiest to get into and out of and easy on the knees; however, they are perhaps the worst when it comes to off-bike footing. Looks, which I used for many years, are pretty easy to use once you figure out the technique that works best for you. A few years back I switched over to Campy's ProFits which are, for all intents and purposes, use the same basic design as Look with the Look pedals I found it was easier for me to kick my rear heel "IN" to get out of the pedal instead of "OUT". The Campy pedals are the opposite, where it's easier (and more natural) to kick my heel "OUT" to exit.