Following a TO the ball is bounced to A1 for a spot throw-in on the backcourt (endline). Before the ball is caught by A1 he runs inbounds & A2 simultaneously replaces him & catches the bounced ball. What do we have?

I know the thrower (A1) cannot be replaced on a spot throw-in once the ball is at his dispossal. But isn't dispossal defined as the ball being caught by the thrower when bounced to him?

Help me out, thanks!