It isn't that hard to picture in your mind. The cable and the brakes are in series so the cable travel with be double that of a single caliper. It has to pull both of them closed in series before they contact the rim. It'll have 2x the travel of just one cable and also 2x the leverage ratio. But it will have 2x the swept braking area so it'll have approximately 4x the braking power (if the lever has enough travel). One could change the lever ratio slightly to overcome the travel issue and the fact that they might be a bit grabby.