You can not join over the different databases. If you do not have thousends and thousands of users, you could just map them into PHPCR. Otherwise you could do two queries. One on the ORM to fetch your user(s) and then one on PHPCR-ODM to fetch all posts that have a username that is one of the users you are looking for.
For links from ORM to PHPCR, you can use the "referenceable" feature of documents and store the UUID in the relational database. You can find
a document by UUID:
$dm->find(null, $uuid)