The tnsnames editor mixes up existing entries, when the 2nd alias contains an underline.
When opening the editor the properties of the 1st database have the hostname of the 2nd Database.
SQL Navigator does a partial search of the instance name instead of exact/full string search.
WORKAROUND:
The work around for now is to have the shorter name before the longer name in the tnsnames.ora
STATUS:
Waiting for a fix to be released in a future version of the SQL Navigator for Oracle.