PG -
I run into this issue all the time (people working different shifts sharing the same desk, pc, and phone) and they both want MWA.. [
I was looking for that emoticon that show the happy face banging his head against the wall..- That's a good one.] 
Short answer, I haven't found a way to do it with MerMail or Centigram (That's the VM system I have)
So I usually place both dn's on a 2616 with Key 0 marp'd for the first user and MWA for the first user. Then I marp a second dn on key 8 for the second user and give him/her pager notification when a new message is in.
When asked (
and they almost always ask) why I can't activate the MW light for both DN's, I say
"MW
status is based on a DN basis, but the switch sends the
on/off states of the MW light to the set - not the DN - So, even if I marp both DN's - the set will only respond to the last on/off command generated by the switch - Thus the light has no relevance to the New or Checked messages in the mailboxes - The MW light (in this case) only indicates that MB A or MB B has checked their messages and that there might be a new message in one of the two mailboxes ---- So who wants to be paged when a new message arrives in their mailbox?"
I'm not sure if that's all true, but it makes sense to me....
Chas