Generally I would say it is a vary bad idea to give the stoker control of a brake.
Hmmm. Probably not a universal rule since there are so many variables, e.g., seasoned cyclist-stokers, experience, good communication or just that ESP tandem teams who have ridden together for many years seem to share. However, if you find yourself riding with a variety of stokers who have varying degrees of skill or experience then, yes, if I didn't keep control over it I would agree that at least some special instructions might be required, such as "I'll tell you when to apply and release it".
Now, here's the interesting part. If your rear brake cable runs down the side of the top tube your stoker already has the ability to apply the rear brake. All they need to do is simply reach down, grab and pull up on the exposed brake cable and voila, the rear brake will be applied... Just something to think about.
Us, well yes I do have control of the rear brake on our tandems. Debbie wasn't a seasoned cyclist when we started riding so it was an easy decision early on. I've kept it that way since it's so rare that we use it that it would be difficult to actually develop our "team braking skills". However, if we lived in the mountains and used it more frequently I'd probably have set it up for use by Debbie.