The home plate umpire has ALL runners at third and the runner at second with runners at first and second.

The base umpire has ALL runners at first base, the runner at 2nd when runners are at 2nd and 3rd

The runner at 1st and 2nd when the bases are full.

As you can see, it IS the Base umpires responsiblity to take care of tag-ups.... and you CANT do it without keeping an eye on the ball