I have not done this exactly, so these instructions might not work perfectly, but here's the basic concept of how it would work.

The functionality in the client that allows for options in a dropdown to be populated based on another field/value is called Dynamic Fields.  This allows a database to be set-up with a list of options for Field A that filters the list of options made available for Field B.

If you have never used Dynamic Fields before, you may want to get your VAR involved to help you set it up.  EDIT TO ADD: I just noticed you are a VAR, haha - sorry.

This is going to require a database you can set-up a View on - preferably one that already has a list of your users, so that you can just create a View from that existing Table.

Your database view will need to list all possible combinations of users between Field A and Field B, excluding when Field A = Field B.  You probably also need to include all options for Field B when Field A is blank.  The query for that View might look something like this: 

SELECT '' AS FieldA, e.employee_name AS FieldB
FROM employees AS e

UNION ALL

SELECT e1.employee_name AS FieldA, e2.employee_name AS FieldB
FROM employees AS e1
LEFT JOIN employees AS e2 ON e2.employee_name <> e1.employee_name

That should give you a list something like this:

Field A             Field B
------------------  ------------------
                    Employee One
                    Employee Two
                    Employee Three
Employee One        Employee Two
Employee One        Employee Three
Employee Two        Employee One
Employee Two        Employee Three
Employee Three      Employee One
Employee Three      Employee Two

On the template, setup Field A based on the Dynamic Field from FieldA with no filters and the Field B based on the Dynamic Field from FieldB filtered based on the selection of Field A.

The initial user will see it without Field A or Field B populated, so Field B should dynamically populate from the Dynamic Fields based on Field A being blank (that's the first half of the query where it lists FieldA as blank instead of an actual value).  This is the part I'm not entirely certain will work as expected and you might need to have Field A populate via a token of the current user instead of being blank.

On your workflow that is moving the document to the selected folder, you'll have it used Field B to route it and move the value from Field B to Field A, clearing the contents of Field B when it does it.

I hope this helps.