See the above figure, there are two blocks and they are kept on table with contact each other, then Newton's third law is applicable between two blocks and with table. Here we see external is also applied on the block 'A'. This same force 'F' is also automatically applied on the block 'B' because it contact with block 'A' i.e., force 'F' is applied on (m1+m2). Therefore, acceleration
Between two blocks A and B, we can apply Newton's third law is also applicable, therefore
f2=-f1
If you consider the block A alone, then the net force acting on A is F-f1, therefore, acceleration
If the force applied on the block B, then the common acceleration is same but the contact force will be different.