For i As Integer = 0 To listbox1.Items.Count - 1
    w_string = listbox1.Items(i).ToString
    s_string = listbox1.Items(i).ToString
 
    w_string = Split(w_string, " ")(0)
    s_string = Split(s_string, " ")(1)
 
    zug_weiß.Add(Trim(w_string))
    zug_schwarz.Add(Trim(s_string))
Next i
 
'Oder so:
 
For i As Integer = 0 To listbox1.Items.Count - 1
    Dim w_string() As String = Split(listbox1.Items(i).ToString, " ")
 
    zug_weiß.Add(Trim(w_string(0)))
    zug_schwarz.Add(Trim(w_string(1)))
Next i