Kommt unter DieseArbeitsmappe im VBA-Editor.

Option Explicit
Option Compare Text

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)

'# zu durchsuchende Tabelle referenzieren

  Dim wksSearch As Excel.Worksheet
  
  Select Case Sh.Name
    Case "Transporter"
      Set wksSearch = ThisWorkbook.Worksheets("Termine")
    Case "Termine"
      Set wksSearch = ThisWorkbook.Worksheets("Transporter")
    Case Else
      Exit Sub
  End Select
  
  If Intersect(Target, Sh.Range("B:C")) Is Nothing Then
    Exit Sub
  End If
  
'# Name und zu durchsuchenden Bereich ermitteln/referenzieren
  
  Dim rngSearch   As Excel.Range
  Dim strFullName As Variant
  
  strFullName = Trim$(Sh.Cells(Target.Row, "B")) & "," & Trim$(Sh.Cells(Target.Row, "C"))
  
  If Left$(strFullName, 1) = "," Or Right$(strFullName, 1) = "," Then
    Exit Sub
  End If
  
  With wksSearch
    Set rngSearch = .Range(.Cells(.Rows.Count, "B").End(xlUp), .Cells(.Rows.Count, "C").End(xlUp))
    If rngSearch.Row >= 2 Then
      Set rngSearch = .Range(.Cells(2, "B"), .Cells(rngSearch.Row, "C"))
    Else
      Exit Sub
    End If
  End With
  
'# Suchen
  Application.EnableEvents = False
  With rngSearch
    .EntireColumn(1).Insert xlShiftToRight
    
    .Columns(0).FormulaR1C1 = "=CONCAT(TRIM(RC[1]),"","",TRIM(RC[2]))"
    If .Columns(0).Find(strFullName, , xlValues, xlWhole, xlByColumns, False, False, False) Is Nothing Then
      strFullName = ""
    End If
    
    .EntireColumn(0).Delete xlShiftToLeft
  End With
  Application.EnableEvents = True
  
'ggf. Ausgabe
  If strFullName <> "" Then
    MsgBox "Der Name '" & Replace$(strFullName, ",", ", ") & "' ist bereits im Blatt '" & wksSearch.Name & "' vorhanden.", _
            vbExclamation
  End If
  
End Sub