I've seen drum brake/rim brake combos on tandems (And triples for that matter), so it's certainly possible to have rim & disc brakes (Given you have a frame/fork and hubs that are set up for disc brakes - if not, consider SRAM's drum brake). On drop bars you could add a mountain bike/bmx lever on the flats for the second brake. On a straight or riser bar you'd just mount a second brake lever somewhere on the bar.*

But why stop there. What happens if you have a flat - It might be possible to lace up dual rims so you have back-up tires front & rear. And what about the possibility of your frame coming apart - Could you run double tubes along side each other? And what if...

Just kidding - Good luck!?

*Do you own a car? If so, I can't imagine the type of brake system you must have.