Friction Coefficient for Brake Pads: Validity of F=2x(UN) Equation?

[protector]

I've been trying to figure out the friction coefficient of some brake pads. My test fixture only allows me to pull (F2) a brake disc between two brake pads clamped around the disc with a force F1. I then measure with a load cell the maximum force required for the brake disc to slip.

My question is simply, will the basic equation F=UxN be valid, or is it correct to assume that F= 2x(UN) in this scenario assuming F1 and U is identical for both brake pads?

I've been googling the topic and my impression is that both equations are used.

 

[rcgldr]

Should be F=2 x (UN). Imagine replacing one of the pads with a frictionless pad and then you'd get F = 1 x (UN) for the one pad with friction.