Hi once more @mirkoelettronics, I appreciate all of the feedback and suggestions. I'm not sure if there is any technical limitation here, or just a limit in our UI design. I'll log your request and talk it over with our product/dev teams.
In the meantime, you should be able to accomplish something equivalent by using the Cayenne MQTT API to connect your Raspberry Pi / Arduino rather than the 'Raspberry Pi' and 'Arduino' option in the Cayenne UI.
If you look in that documentation, you'll see we have examples that cover connecting both Pi and Arduino. The plus side of using MQTT here is that you have an effectively unlimited number of channels to send and receive data on.