The way I would handle this is to have the MER Record - MERS Analyst field only show when editing/adding the record. Then create a new formula text or user(this is my assumption) field that will be shown when viewing the record and use this formula. Have the formula field only show when viewing the record.
If([MERRecord - MERS Analyst]="",[PORecord - PO Analyst], [MERRecord - MERS Analyst])
This should result in the user being able to add text to the MER Record - MERS Analyst field when editing or creating the record but it will only display the formula field when they view it which will show whatever field is filled in but defaulting to the MERS Analyst when there is text in both. You may also want to put a little help text bubble on both fields to let people know that if nothing is entered there it takes the text/user from [PORecord - PO Analyst] and uses that.
