Each sound (patch) needs its own MIDI channel
My guess is that both tracks are on the same MIDI channel (probably 1)
So when you change the sound assigned to that MIDI channel it changes both tracks.
You could use Edit-rechannel to change one of the tracks to a different MIDI channel.
Or right-click the track name area and change the MIDI channel.


I do not work here, but the benefits are still awesome
Make your sound your own!