My vintage tandem came without the rear drag brake setup so I experimented for a while using two rear calipers with a brake cable splitter/doubler from the lever. I’d have to say that experiment failed. I found I got better braking, and certainly better brake “feel,” from one caliper. I found my experiments with brake shoes more fruitful for improving brake performance. Conversely, I’ve always found it pretty easy to achieve as much brake performance as I need on the front with a single caliper of any kind. So, other than achieving a unique look (which I can’t discount), I think you’ll find two up front aren’t necessary.