Hi John,
LDAP is a protocol, but implementations like OpenLDAP can distinguish "virtual hosts" based on trailers of the distinguishedName of each query. So, you would setup dc=example,dc=com for one "virtual host" and dc=example,dc=org for another. When you are looking for uid=john,ou=Personnel,dc=example,dc=com you would end up in the first "virtual host's" subtree. In OpenLDAP, it's called a backend database.
-Rick