Prison records are public records. They can be found in several places:
1) the local court where the paperwork went through - there may or may not
have been a trial. Could be city or county court most likely although it
could have been state as well.
2) PA archives. I bet they have records on microfilm somewhere. Their
digital archives can be found at http://www.digitalarchives.state.pa.us/
3) Local newspapers - I bet even in a large city like Philadelphia, bigamy
will make the news. Try http://www.newspaperarchive.com LDS may have some
too. Possibly ProQuest.
4) Local law libraries, you have to know the court and session. If you are
nice to the librarian they will usually help you.

Linda Altman