I kinduv agree re; coaster brakes. Okay on the boardwalk (maybe) or some other controlled environment, but otherwise no. The reason is because when you need to brake with a coaster brake, the stoker also needs to brake (backpedal) at the same time and that requires a degree of communication that probably isn't possible verbally unless you're traveling very slowly at the time and have plenty of time to react. But when you need to stop, you really need to stop.
Learning to communicate (non-verbally) on the tandem is the fun of it, but the intuitive communication required to brake by backpedaling seems like it would be a daunting task. We've been tandeming together for 25 or so years and rarely, if ever, communicate verbally any more (other than 'bump', or sometimes, 'standing'). But I don't think we could successfully use coaster brakes together. Except in a controlled environment like a boardwalk.