After a few days, my PC accepted both sound cards. They are two identical sound cards. Icy Box IB AC527. It has named one with 2- USB Audio Device and the other with 3- Audio Device. Both work independently. Depending on the programme, I can select the sound card I need. So I can have different configurations for different applications permanently connected.
The solution was to give the PC time and restart it every day.

My research has shown that it is theoretically possible to operate two sound cards, but in practice only one can function/be active as the "standard". Switching between them should be possible.

However, as far as I could see, there was talk of two different sound cards. From what I could read in forums, there are further obstacles with two identical sound cards, as they hardly work without problems. :-(

Perhaps someone who has already dealt with this issue and can help me further will get in touch. I have no personal experience, but I hope that I have been able to help.