Calculate combined friction on multiple different surfaces?

[Serilla]

Hello,

I have a cuboid on two different surfaces. I am applying a changeable force F to it, but it should stay on the same point / in static friction. I now want to calculate the combined (static) friction coefficient μ_combined out of the two known coefficients of cuboid to surface 1 μ_1 and cuboid to surface 2 μ_2. Forces (and if I forgot anything else) can be assumed to be known, it's just about calculating the combined friction coefficient.

But how am I calculating it?

My thoughts are that (e.g. mass and area of the cuboid is 50% on surface 1 and 50% on surface 2):
μ_combined = 0.5*μ_1 + 0.5*μ_2

Is that correct or is it too simplified?
Thanks for the help and sorry for my bad english, I'm not a native speaker.

 

[mfb]

It will depend on details of the block and the surface (e.g. tiny bumps in it) that go beyond this simplified description. It will be somewhere between the two coefficients of friction but we can't know where.

 

[jbriggs444]

A torque calculation could put some slightly tighter bounds on the possibilities -- if we had additional information about the point of application of the force.