I'm going to leave my original answer (first paragraph) in place just in case it's relevant, but I'm now thinking it's not the case.
I think because you have a many person to one manager thing going on, you'll have to use a code procedure, otherwise I see any query as being one that returns the manager name several times because of the many persons associated. So if in code you loop through your recordset and build a TO list for persons and a CC list for supervision you should be able to get what you need.
You didn't actually say that you want to include the people who are 'due an event' - just the supervisors. Your query needs to include the email addresses for supervisors that it retrieves from the 'persons due' data. No idea what that looks like at your end.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.