Two man coverage on transition 3

[M&M Guy]

Hire a third official. Oh, wait, you said...

Since you said you were already under the basket, and the T was across the division line, I would say it is the T's call all the way. It should be their responsibility to get into a position to see that foot. Of course, they can look to you for help, but if you're already watching a matchup, you're doing what you're supposed to be doing.

Of course you pre-game this situation, but in most cases the only time the L would call "outside" their area is if they were even with the lead players in transition, while the T is still in the back court.