Dim query = (From tData In tDataMandant1.Union(tDataMandant2) Group By GrpKey = _
  tData.ArtikelID Into MergedData = Group
     Select New DataClass With
     {
        .ArtikelID = MergedData.First.ArtikelID,
        .Text= MergedData.First.Text,
        .Bestand = MergedData.Sum(Function(x) x.Bestand),
        .ZusatzInfo = MergedData.First.ZusatzInfo                      
     }
  ).ToList