I thought these superfabs were very sensitive to ground movement, even with all the measures taken to protect against shock. Would it really be smart to build another fab right beside an operational one that is crucial to your production needs?
The operation of one fab shouldn't generate sufficient vibration to affect another - why would it not also affect its own processing?
What will affect production is any blasting to dig the foundation of the new fab. All sensitive operations and machines would have to be put in a benign state to avoid loss of WIP or even damage to the tools.
BTW, I was once in a facility when a new building was put in right next to it. The contracters could only blast at several fixed times each day and sensitive work in the existing building was scheduled around the "boom times".