What would you do?

[gumpire]

When I gave the rule quote, my intention was not to show that rules would solve the problem.
What I was trying to show was that I think that a discussion between umpires would have appropriate in this situation and, since what you've said is that your partner was out of position, he should, at that point, admit to you that he didn't see the play and defer to you.
If you meet with him, and him alone, and he still won't admit the mistake, the call would stand. I would still discuss it further after the game.