So, I have solved this issue on my own with the help of youtube (as usual).
You can add more than one audio interfaces at the same time in S1 if you select "Asio4all" as S1-audio-interface in the Audio IO settings within S1 ( starting page e.g.). Then you have to activate the respective external audio interfaces in the asio4all setup and select any of the channels, which are provided by the individual interfaces. Finally you will see all those channels available in studio one, independently from the external devices.