The Stages of a Fight.

[Rook]

Can two people involved in an altercation be in different stages at the same time?​

i.e. Guy A is escalating, but Guy B is fighting?​

A: "Hey Mofo, why you looking at my..."
B: WHACK!

Or does guy A automatically move into fighting because B did?

I would just consider that a more sudden escalation. I would say that the fight has now moved from verbal to actual attack, skipping the pushing that comes between in the "standard version."