Mach doch das Ganze mit Script.
Beispielcode für einen Button innerhalb der Maske für Dokument B:
...
Dim session As New NotesSession
Dim workspace As New NotesUIWorkspace
Dim db As NotesDatabase
Dim doc As NotesDocument
Dim docA As NotesDocument
Dim uidoc As NotesUIDocument
Dim uidocA As NotesUIDocument
Set db = session.CurrentDatabase
Set uidoc = workspace.CurrentDocument
Set doc = uidoc.Document
Set docA = db.GetDocumentByUNID(doc.FeldmitUNIDvonDokumentA(0))
If docA Is Nothing Then
Messagebox "Dokument nicht gefunden"
Else
Set uidocA = workspace.Editdocument(False, docA)
End If
...
Axel