again, stop and think about it for a second.
we have 100 psi. that is 100 pounds per square inch.

if we double the square inches, we double the force.

another way to think about it.
assume we have a single piston, and 1000 psi of force being applied to
from teh piston, and further assume that the brake pads are 10 square
inches.
with a single caliper, there are 2 pads, so that 1000 psi gets spread
over 20 square inches, for 50 PSI on the pads. with 2 pistons, you
would have 100 psi, and double the friction.