[nickrego]

Actually, it is the Plate umpire's job to make that Interference call !

With only a runner on 1st, his job is to watch the continuing play at 2nd base for just such an occurance.

The Base umpire's job is to watch the ball and call the plays at 2nd and then 1st.

Remember, two sets of eyes does not mean 4 eyes watching the same thing. We each have a specific job to do for each situation.