precisely, the cable is not anchored to the first caliper, the cable must travel through the caliper and it is the cable housing that closes the first brake, the cable closes the second caliper.
meaning the amount of cable to pull both brakes closed (because the length of housing is constant) must be twice the normal length of cable pull, but given a standard pull brake lever the with the same amount of travel we can assume that caliper travel will be cut in half.
As long as the pads are close enough to still reach the rims both will actuate with equal force (provided identical calipers) and since the force between the two brakes is equal we can ascertain that to reach nominal braking pressure twice the force would have to be applied to the brake lever.
Not to equate braking force with stopping power as the amount of braking area has doubled as well.