I’ve just tried it and it doesn’t change the number in the other channels gain level on the other channel but other than that i thought of using the trim on one and the gain on the other as you said
If you want completely independant control of BOTH chanels you probably want to first set the preamp gain to the right ball park, then set BOTH chanels to be trim on surface. Then you can adjust the trim on either chanel without it affecting the other.
If you have one channel with gain on surface and the other with trim, you can adjust the latter without affecting the former, but if you adjust the gain on the former it will affect the signal level on the latter, even though the trim value won’t change. That MAY of course be what you want in some circumstances.