Hi Marco, I had a similar situation, where we needed to get around different settings between C/L Band readings using the same physical Decimator.  To get around the setting differences, we created a second DataMiner element of the same physical SED Decimator, different element name pointing to the same Decimator IP.  This should act as a second user instance to the Decimator and allow different settings for the session.  The downside, if viewing the Spectrum Monitors live, you will see the trace cycle through the Decimator 8 inputs during the data collection period of the monitors.  However, once the monitor cycle is complete, the current user settings should be present again.  Give this a try and let us know if it solves your issue.

Additional Information - The image below shows the single Decimator updating two different Measurement Point traces. The system time cycles between the two users displays.  On the trace screen the users are able to see the there are multiple clients active.  As you can see, these were done from the same physical Decimator element.  One caveat I noticed, was that if there are monitor scripts running, sometimes, the trace display will switch to the other input reading, but then comes back to the selected one when the monitor scripts are finished.