Two Umpires System question

[Antonella]

Need some help about this.
I am aware of what the Manual says.
Now someone is saying it could be changed...

No runners on base. B1 hits a short ground ball to right centerfield.
BU moves to foul territory, ready to judge the play at first base.
The subsequent throw is not correct and the runner go on towards second base.

Wich umpire will cover the play at second?
And what if in case the runner will go over third and home plate?

Thanks everybody (if possible pls cite ASA or other ruleset...)

Ciao