This situation can be kicked around for ever, with no "GOOD" solution, to have every base & sit. covered.
Communication w/partner and a good pre-game w/partner is essential requirements.
What it comes down to it, there are only 2 umpires, and you cannot see "everything". Some may say this is a excuse, but you can only watch so many things at 1 time.
Now if you had eyes like the actor Marty Feldman, (deceased) you could see both the 1st base line and 3rd base tag up.
Good luck.