I have only been with the University I am at for a few weeks, I have been able to make Admin changes in Canvas for a matter of days and I have found the process for cross listing courses falls short of being intuitive. Maybe it is just me, but after some short conversations I see that there may be something to improve.
To me the intuitive move would be to add the "sub" course to the "master" course from the master. I can see from a development POV that the sub course initiating the change could be easier for testing, but that isn't intuitive to an end user in my opinion. I have spent some time this morning poking around with API calls that are available and it seems from a cursory take that it shouldn't be too much work to build out a quick tool to do this.
I searched for a similar topic here, but I have yet to find something. If there is an ongoing conversation, I would appreciate it if someone could point me in that direction as I am just finding my sea legs in the Canvas community.