Okay let's say your pushing a ball  ,that ball itself can be composed of numerous more balls  ,but you only take the net force an the entire system i.e the ball as a whole  .

Similarly you can assume the blocks in contact to be a singular block whose net force is F and acceleration is F/m_1+m_2

Okay let's say you're pushing a ball, that ball itself can be composed of numerous more balls, but you only take the net force an the entire system i.e the ball as a whole.

Similarly, you can assume the blocks in contact to be a singular block whose net force is $F$ and acceleration is $F/(m_1+m_2)$