I have two independent servers, with both variations, both work.
---- moduleload back_bdb moduleload back_monitor
backend bdb database bdb suffix "dc=xyz" directory "/var/lib/ldap" rootdn "cn=Directory Manager,dc=xyz"
database monitor rootdn "cn=Administrator,cn=Monitor" rootpw foobar --------
-------- moduleload back_bdb moduleload back_monitor
backend bdb database bdb suffix "dc=abc" rootdn "cn=Directory Manager,dc=abc" directory "/var/lib/ldap"
database monitor rootdn "cn=Directory Manager,dc=abc" -----------
Note I didn't specify suffix for monitor database. In 2nd case, credentials for monitor rootdn are taken from other db.
P.