I'm confused about the motion on two blocks sliding one over the other when more that one force is acting on them.

Consider two blocks $1$ and $2$, with different masses, with $2$ placed over $1$. There is friction between the blocks.

Suppose that two different forces (of a generic magnitude) $F\_1$ and $F\_2$ are applied to the two blocks in opposite directions, in the picture that is situation $(1)$.

What will the forces of friction on the masses be?

[![enter image description here](https://i.stack.imgur.com/PVQuo.png)](https://i.stack.imgur.com/PVQuo.png)

I included two diagrams $(2)$ and $(3)$ in the picture. In $(2)$ there is only one action reaction pair, called $F\_A$, due to friction between the two blocks, but is it the only one?

I mean the blocks are trying to slide in opposite directions and each one tries to bring the other behind, so shouldn't there be a second pair of forces, due to friction too, as in diagram $(3)$, where it is named $F\_B$?

If the correct diagram is $(2)$ how can one determine the directions of the only one pair? If I got that right, friction (the pair of forces in question) is always opposing relative motion between two masses. So must I determine the direction of relative motion first and then draw the pair in the opposite direction?