You can do this with rules. Your rule needs to use "filters" to get the correct row from the 2DS (secondary data source).
So if you have a table with a row like ["Fred Flinstone", "Quarry Worker", "[email protected]"], your rule would look up the 2DS row where the Employee Name matched the name selected in the field that the drop-down was attached to (i.e. the filtering feature lets you do that compare) and then select the email from that row to assign to the other field.