I found this out recently, that if you create a NotesViewEntry from one view, you can only add entries that exist in that view. So you can't combine entries from two different views.
A possible way round this would be to use a java.util.TreeMap, push the entries into the TreeMap with the date as the key. This may work, but you may need to convert the NotesViewEntry objects to your own non-Notes objects before adding them in. This will definitely be the case if you want to store them in a managed bean of session or application scope. No matter how you store them, if you use a TreeMap it will have a performance hit if you're dealing with a lot of entries.