By the Tip of "011111100010" I found this as my Solution :
arg1 = 9
arg2 = 6
arg3 = Application.Run("'Test-Mappe2.xlsm'!Modul1.Add", arg1, arg2)
For testing I wrote a Function called Add which is placed in Modul1 inside my 2nd Workbook (Test-Mappe2.xlsm).
This Function has to add both Arguments and deliver it's Sum back.
The Method Test is placed in my 1st Workbook (Test-Mappe1.xlsm).