Another issue that can turn one upside down is when mbx A has a different extension than the mbx number. This is found in CDR using MWI event (I think 18).
For this scenario, I presume you want to light MWI on 2 separate extensions, at the same time. If so, this can only be done through switch translations, as far as I know. But I am open to learn how it can be done if it gets done.
Pat