Conditional Dropdown using Values field change

I know how to use the Conditional Dropdown behavior to make the select list of Dropdown B dependent on the Value chosen in Dropdown A.
But this cannot be used for my situation. However, I am hoping someone in the PHPRunner community has already solved this kind of dropdown.
I have a master table of pharmacies. There may be 120 pharmacies all containing the same value in one field which indicates that one particular person services all of those pharmacies.
So let's say there are 30 different service personnel. In the pharmacies table there is a field that holds their full name, and another field that holds their key id number.
When "Sarah Brown" logs in she is given a filtered list of pharmacies, only permitted to see those pharmacy records which have her name and key id #.
The solution I am looking for--
In a Reporting form I have a field where one of the Service employees is selected from a dropdown list of 30 people.
What I then need is to provide a dropdown list B which dynamically changes according to the Service employee who is selected above this field.
In the dropdown list B of one pharmacy to choose from I want to provide only the selection list of all pharmacies belonging the selected Service employee in Dropdown list A.
So, unlike the Dropdown Dependency of PHPRunner, this dropdown must be assembled by a filtering action that does not load (so there are no errors) until a value is entered into Field A.
the WHERE statement would be " WHERE service_name = '" . $value['fieldA_value'] . "'";
I have used a Javascript snippet to search a table for the value entered into a form before the form has been submitted and notify if a record already exists.


Is there a way to use the "live" SET value of Field A to immediately compose a dropdown list using that same value in Field A that can be accessed in Field B?
Thank you for taking the time to read!