The exercise is basically this:http://prntscr.com/4e14nfI'm having difficulties in getting to the free body diagram for the block 1 when both blocks move with the same aceleration = a.Is there 2 friction forces o the surface that connects 1 to 2, opposite to one another?And does each one appears on the movement equation of the specific block?Thanks in advance.

Your link is broken, but I'm guessing this is about one block sitting on top of another, correct? Friction acting on block A due to relative movement of block B is equal in magnitude and opposite in direction to the friction acting on block B due to the relative motion of block A.

Is there something in the problem statement that says both blocks move with the same acceleration? That would be unusual, as typically in these types of problems the blocks move relative to each other - otheerwise there is no friction force to worry about, which sort of undoes the whole purpose for making you work through the problem.

EDIT - I figured out that he correct URL is: http://prntscr.com/4e14nf Indeed, the blocks don't have the same acceleration.