What's wrong with the two-umpire system?

[FUBLUE]

I worked a national recently and we worked 2-person all the way through the tournament. I saw many situations where two person mechanics fail to get the best coverage. I was wondering what you all have found to be faults of the two-person system and if you have suggestions to correct the coverage.

Here's one: R1 on 2B, line shot to F9 with possible throw to F3. BU is watching the play at first, PU is watching the hit ball. Who's watching R1 touch/miss 3B? (A coach in this area teaches his kids in this situation to miss the base by about 3 feet on this situation because he knows no one is looking at the runner).