Two guys simultaneously shoving each other for position. Either call it on both or call it on neither.
From SI:
"Mutual combat" in football is typically a term used to describe a receiver and defensive back being "mutually" as physical with one another on a play, meaning no penalty would be called despite that physicality.
With the receiver and defensive back competing for the ball, officials sometimes leave potential pass interference as a no-call because of how entangled both players are in the interaction. If one player is blatantly interfering, that changes things, but "mutual combat" is more about reciprocal hand-fighting reflected on both players.
We'll agree to disagree.