Pascal's law says that the hydraulic pressure must everywhere be equal. Pressure is defined as a force devided by an area.
P = F/a. Thus the force
F is equal to the pressure
P times the surface area
F = P . A
Consider a simple brake setup with 1 master cylinder and 1 caliper (with 1 pistion). The pressure at the master cylinder must equal the pressure at the pistion of the caliper. If the area of the caliper's pistion is twice the area of the master cylinders pision, the force on the brake disk is double the force applied to the master cylinders pistion.
Now if you add more pistions to the caliper, the total surface area increses, thus the force applied to the brake disk is multiplied even further. Adding another caliper is effectively the same as replacing the caliper with one that has more pistions.
The only practical problem you may get is the volume of brake fluid required to move all pistions. The amount of brake fluid required to move all pistions is equal to the distance a pistion moves before reaching the brake disk times the total surface area. For example:
Each pistion moves
2 mm before both brake pads are touching the disk and you have 2 calipers (1 pistion each) with a total surface area of
25 cm^2, the amount of fluid required to move the calipers =
0.2 cm * 25 cm^2 = 5 cm^3 (cc). Fluid is not compressable, so after contact is made, no more fluid is needed to apply more pressure.
Obviously the master cylinder must be able to move the required amount of fluid in one go for the extra caliper to have effect. Else you have to upgrade to a bigger master cylinder.