We actually have a working version of a disc / V set up for a tandem... the brake timing has to be very precise and the front disc acts like a front drag brake and will also bleed off a great deal of speed before the V brake engages to add that little extra bit of wheel stopping power.

It is set up like a lever operated rear drag brake / rim brake combo as with this you want the drag brake to engage with the initial light pull and then have the stronger rim brake engage with the drag brake with a full pull of the lever.

This would not even be needed on a single bike as a V brake or disc has enough stopping power to toss you over the bars but on a tandem that might be descending a mountain at 80 kmh with 500 pounds on it, more is better.

It also adds some redundancy to the system as the two brakes are operated with a single lever and have a proprietary linkage... it is very simple to re-rig the bike to use one of the single front brakes if needed.