Vedeja I think you can only select 1 midi command per track, if track 01 mute is CC22 for example, if you choose track 02 and assign the same CC22 to mute both, it erases the midi command on the first track and add it only to the second.
That’s the whole point on having mute groups.
Hopefully we have control over mute groups soon, also it would be great to assign multiple actions to the same Midi Command.