A trial judge in Bucks County can sequester witnesses either on their own or by motion of the prosecutor or Bucks County defense lawyer.
The theory behind sequestration involves common sense. A judge does not want one witness influencing another witness with their testimony.
Once sequestration is ordered, it must be enforced by the prosecution. If it is not enforced properly, there are remedies available by the judge. I once made a motion for a mistrial because it was my belief that two officer's inappropriately communicated about planned testimony while sequestration was ordered. Specifically, I felt they were matching up their stories/comparing notes to testify to the same course of events. The judge never ruled on my motion for a variety of reasons, including the fact that the Commonwealth withdrew its prosecution.
The only time sequestration (or lack thereof) is reversible is only if there is clear abuse of discretion. In general, I have never seen a judge have a problem with order sequestration as it just simply makes sense.