You would appear to have a problem right at the start here:
Two databases, linked many to many by medical record number (MRN).
How did you relate these two tables? If you used:
db1::MRN = db2::MRN
You don't have a many to many relationship, you have a one to many relationship and this might be at least part of the problem here.
For a true many to many relationship, you'd need either a join table linking these two tables or possibly a multi-value key in either db1 or db2. (I don't recommend the multi-value key for this.)
A search where you enter "ortho" in the text field of a db1 field and a lone * in db2::MRN should find all records with ortho in the text field and where at least one related record in db2 exists. This can be done in a single find request.
That said, the criteria you describe should also work as far as I can tell without running a few tests.