What about two levers for the captain and two levers for the stoker? I've always found that the rim brakes are enough for any kind of instantaneous braking needs, but on long downhills when you've built a lot of speed sometimes those additional brakes can come in handy to keep your rim brakes from overheating. Just ask your stoker to go ahead and put the brakes on....has worked for me.
IMHO, two rim brakes and a drum in back are very nearly perfect - with the stoker controlling the drum.