To help on this could you post a screen shot of your Song routing so we can see what you have going? It helps us to troubleshoot. The video you point to is about keyrange splitting so I am confused as to what you refer to about “channels”. Do you mean MIDI channels?
Yes I was talking about MIDI channels.
So my problem is, on this example, that my trombones are on midi channel 1 and my string on midi channel 2, but despit of having selected the midi channel 2 to play the string, it’s my trombones who are playing …
How are you selecting the MIDI channel? Your keyboard controller?
Anyway I checked the video and what Brad is doing is using single slots in Kontakt set to channel 1 and Pseudo Presets to switch between them. What you have is a Kontakt rack loaded with 2 instruments each set to different MIDI channels (1 and 2). To switch between them you need a different approach. I would recommend using states (Song States in this case). First you need to create the first state by going to the left “States” panel and clicking “New State”. Name it Trombone.
Save the song and now you can switch between the 2 sounds using the states panel. Note that the states panel will react to PG changes if you want to use the controller keyboard to change. You can also play both channels at the same time (layered) but you would need to add another state and an input route the points to the channel you want to sound. That setup would look like this