The Duplicate Constituent Merge plugin cannot merge a BBNC related record to another BBNC related record. You can however, merge non-BBNC related records to a BBNC related one. Also the BBNC related record cannot be the secondary record 

For example, the following records will merge:

John Smith - BBNC  linked (master record)
John Smith - non-BBNC linked (merge record)


While the following will not:

John Smith - BBNC linked (master record)
John Smith - BBNC linked (merge record)


In order to merge, the secondary (merge from or merge) records must not be associated with BBNC. The primary (merge to or master) record can however be associated with BBNC.