I just forgot that the AES/SPDIF input feeds digital and that the analog signal would simply be burnt

No, you cannot use 2 RCA to 1 TRS (stereo jack) cable!  You need 2 RCA to 2 TS (mono jack) cable.

Please take a moment to inform yourself by using Google to look up the difference between analog unbalanced and balanced connections.

Quick summary upfront...

Your cassette recorder isn’t a high-end studio model, which is why it has the standard unbalanced RCA connectors commonly found in consumer devices.

Recording interfaces like the UCX II usually support both balanced and unbalanced connections on their analog inputs and outputs. However, these are still mono inputs.

If you use an adapter cable to combine the two mono channels from the recorder into a TRS plug (with three poles) and then plug this into a mono input that supports both unbalanced and balanced connections, you’ll end up with a mess.

A balanced connection is a mono, three-pole connection consisting of ground and the audio signal in opposite polarities (+ and -). This opposite polarity is used by a differential circuit to effectively cancel out noise, such as electromagnetic interference. In plain words: the circuit compares the positive and negative signals and subtracts one from the other. As a result, any interference that affects both conductors equally (common-mode noise) is filtered out, leaving you with the clean audio signal.

If, however, you send two different audio signals (e.g., stereo) instead of opposite polarities, the differential circuit cannot function properly.

Another important difference between balanced and unbalanced connections is that balanced connections operate with higher studio-level signal strengths to enable longer cable runs without signal degradation.

So .. frequencies could be canceled out and also the levels do not match between output and input in your case.