Still a little further...

Now let us consider 2 plates one being above the other, always presumably insulated on the sides, with conductivity k₁ and k₂ , thickness L₁ and L₂ but with the same surface area :

For each plate, we independently have :

T_i being the temperature of the interface between the two plates and R₁ , R₂ thermal resistances of each plates.
Summing both, we obtain :

We then obtain a relation which is valid for the two plates and write

It works like in electricity for resistors in series, resistances are added !