Timing modifications done inside a multi-channel segment are saved in the source clip in the library. All instances of that source clip are updated to use the new timing.
You can only slip a shot to adjust its timing. You cannot trim.
To protect the structure of the source footage, it is not possible to add dissolves or layers inside the multi-channel clip. Use matte containers for more complex editing.
To create a temporary grade to help you set the timing of a matte in a multi-channel clip: