I may be getting confused here but i think we've done this here using DEE (i think i even found it on this site)
Create a user for Ext A, then go into the newly created user, enable Dynamic Extension Express, then click Associated Destinations.
From there right click and select Add Associated Destination.
Change the Type to Desk2 and change the Destination to Ext B. (you may need to change MWI/Pullback to yes if its not already set)
Hopefully this works for you, we have an office with a shared VM box for 3 devices and we use this to make two of them notify of a message at the same time, can't seem to get all 3 to do it though!