Anyone know anything about disc brakes?

There are a couple of important points to mention on the subject of tandem braking, which I have been learning recently having nearly finished my tandem.

1. If your tandem has disc brake mounts them go hydraulic if you can! Apart from the fact that their performance is better in various ways, the most important thing is that they are more "powerful". By that I mean that the ratio of lever-squeezing pressure to braking force is relatively low. That brings me to point number two...
   
2. If you must keep it mechanical, and you're thinking about which is more powerful, then the most important thing to consider is the combination of lever and calliper. Both components will be designed to operate with a specific 'mechanical advantage'. I can tell you for a fact that Spyre callipers do not work well with 105 levers, as this was a common problem with mid-level road bikes at my shop. My advice is to a) walk around a bike shop and squeeze some levers and b) read some reviews.
   
3. As has already been mentioned, a drum brake would be preferable. On the rear that is, although this would necessitate a specific hub. The reason that drum brakes were popular with tandem teams in the pre-disc brake era was that they dissipate heat very well. Consider this marvellous article: https://www.sheldonbrown.com/tandem-brakes.html
   
   

Ultimately, there's two of you so you need more stopping force to match the extra weight. I reckon hyro-disc is the righteous path.

Hope that helps.