And how are the underlying tables/relationships constructed?
The constant address can be text added directly to the layout. Or it can be text stored in a global field (See the storage tab in field options) or it can be an address from a related Tutorial: What are Table Occurrences? that is linked the layout's table occurrence with the cartesian join relaitonship operator.
Your two addresses could be displayed with two records--one for each person in list view, in a portal (think MS Access subform) or you can have two different relationships keyed to your drop down list in order to display selected address data on the same record from the layout's table.
Apologies for not spelling this out in the detail requested, but given the large number of possible wayst to do this, it would take way too long to go into that kind of detail with every option. I suggest describing what you want to take place on your laout in greater detail so that we can narrow the list of possible "best approaches".
I saw your youtube video "conditional list with a twist". I think what you talked there is applicable to my case if I get your concept correctly.
The only videos I have ever made are home movies. Maybe you are referring to those by DaSaint--a frequent and generous poster here in this forum.