Model:
public Class ActionDuengen {
    public ActionDuengen() {}
...
    public string Name {get; set; }
[OneToOne]
public ActionBuchung Buchung {get; set;}
}
public Class ActionBuchung {
    public ActionBuchung() {}
...
    public string Datum {get; set; }
}
View:
...
DuengenAction = new ActionDuengen();
BuchungsAction = new ActionBuchung();
DuengenAction.Buchung = Buchungsaction;
this.BindingContext = DuengenAction;
...

var datumLabel = new Label { Text = "Datum"};
Entry datumEntry = new Entry();
datumEntry.SetBinding(Entry.Textproperty, "Buchung.Datum");