What do you have?

[just another ref]

Originally Posted by Lah Me

And remember that immediate retaliation is always part of a double foul no matter when you blow your whistle. The first foul called during a live ball (personal for contact...technical for non-contact) determines the type of double foul to be called.

This one, as far as I'm concerned is waaaaay out there. There's all kinds of ways to immediately retaliate for something. B1 fouls A1. The ball is now dead.
For whatever reason, A1 is upset and immediately retaliates. This retaliation could be anything. A1 might push him, punch him, curse him or spit on him. But none of these would be a double foul.