Have you tried using the JobNumberExt field in the FindFirst code?
rs.findfirst "[JobNumberExt] = '" & me.cbo_JobNumber.column(2) & "'"
Another way to handle this would be to display the unique job number in the combo box. Then, in the combo boxes after update event, check to see whether there is more than one [JobNumber] with that value. If not, then use the FindFirst based on the JobNumber. If so, popup another form that displays the JobNumberExt to allow the user to select the correct one, then use that field in the find first.
Another method would be to add a FindNext button, and use the FindNext method.