Champion Tactics with GM Wolff - Double Attacks
A "double attack" occurs when two pieces make two attacks at the same time. The difference between a fork and a double attack is that a fork is one piece attacking two things at the same time, while a double attack is two pieces making two attacks at the same time. The trick to seeing this possibility in any chess position is to look for any situation where one piece is "covered" by another piece, where if the piece where to become "uncovered", it would create a threat.
Chess.com Team