In chess, what is the name for the tactics when a piece makes an attack by itself but also opens line(s) of attack for a friendly piece(s)?
The tactic in which one piece moves out of the way of a second so that the second can attack is called a discovered attack (and more specifically a discovered check if it results in check). The piece which moves may or may not move to a position to mount its own attack -- I'm not sure if there's a term for the specific case in which the piece that moves does mount an attack by itself.

You're correct that it's not a fork, which occurs when one piece simultaneously makes two or more attacks. Here you have two attacks but by two different pieces.