I initially considered to connect the two drain tubes of washing machine and dishwasher on a tee, but after research I came up with reports that washing machines do not necessarily have a backwater valve and so the dishwasher water would run into the washing machine. I did not want.
With this fitting I could now connect both drain hoses together to one terminal of the siphon. Test run was successful, everything is tight and it works perfectly. In the connector is up to the transition into the siphon a "partition" between the two Ablaufschlaufanschlüssen ... thus no water is pressed from one tube to another. A good solution.
Note: The black gasket, which can be seen in the picture, was missing for me. It fits a standard seal of the "ordinary simple" connections.
