I was able to piece it together. Included a separator between Data1 and Data2, and specified sortorder for each, and removed blanks created by trailing comma.
Dropdown.Items =
Split(Left(Concat(Data1, email & ","), Len(Concat(Sort(Data1, surname, SortOrder.Ascending), mail & ","))-1) & ",-------," & Left(Concat(Sort(Data2, surname, SortOrder.Ascending), mail & ","), Len(Concat(Data2, mail & ","))-1), ",")