Doug wrote:
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.

You are not doubling the area because the force is colinear and shares a
common ground. Because of this the force cannot exceed that generated by
one piston. If it did it would drive the other piston back into its
bore, but it cant because it is in equalibrium.