Some people call the brake handle between the drivers and passenger seats in most cars an 'emergency brake'. It's actually a 'parking brake' and quite useless in a real emergency. Don't be taken in by Hollywood! Even in those accidents where brakes have failed, especially the ones where investigators found the brake pedal fused to the firewall from the force of the drivers foot given superhuman strength in their panic. In not one of those fatal crashes did the drivers ever try and use the parking brake. When you need to stop, you usually need to stop. NOW. And by using the motions built up through practice and only those motions! There isn't any chance to call for more brake from the back or passenger seat even if there was some there! Either it makes no difference to the outcome (wasn't necessary) or you crash because the primary brakes were insufficient. I haven't personally found found that a single front brake isn't enough for a wide variety of stopping situations. The rear brakes on all our tandems aren't up to much! The primary commuter had the rear brake out for months and we did just fine.
Giving the Stoker control of a proper drag brake is a time honored tandem practice. But it isn't the only way, or even the best way to do it. We briefly owned a Burley Samba with an Arai drag brake. It was set up with the drag on a thumb shifter on the left hand side. We never got to really try it out but I got the idea of how it works and I can't see why any Captain would feel the need to hand off that particular assignment. Stoker appeasement must play some part in things. Some tandem couples think its because my Stoker is blind why I get away with the crazy descent speeds we use when we've been on outings with the tandem club. Nope. 'J' is pretty well aware of how fast we are going and it's TRUST that if I didn't think 45mph was a safe speed for a straight line downhill with a wide shoulder then I wouldn't do it.
The o.p. and his Stoker are a relatively young team in terms of riding twogether. I find it problematic that they found twin piston 180mm discs were so insufficient that they went to four piston 203mm and are still underwhelmed. We are agreed that a v-brake cannot be used in true drag brake fashion where the brake is applied via a thumb shifter, NOT a brake lever, and it is not released until the descent section levels off. There would be plenty of time to warn the Stoker of the upcoming downhill and then tell them to release. The v-brake would need much different management and only the Captain could know how hard and when. It simply is not possible to communicate the finesse of it verbally.
It would be better, I think, to use the v-brake as the primary rear brake (it's been done!) and put the 203mm disc on a thumbshifter and use that as a drag brake. I'd still want to have control of it, but in a scenario like that I wouldn't have too much misgivings about letting the Stoker play with it. I keep my Stoker entertained by letting her navigate. As I said, blindness does not prevent her from knowing exactly how fast we are going, and on a club ride she is about as good a GPS as there is. In town she is even better. She rarely misses a turn. No car has ever been set up with dual controls except the Student Driver jobs. Even so. People have been taught how to drive in standard cars where the instructor had no override. No wonder it usually goes so badly. As I write this I am recalling that only a couple of months ago a student pilot and his instructor were lost in a fatal crash of their Cessna. Dual controls? Absolutely, but the scant microseconds it must have taken for the more experienced pilot to perceive danger and intervene were not enough. Some things you need to stay on top of to stay safe. I'll leave it there.