Hi Marc,
If you are not a Moodle Developer then you will not be familiar with Moodle Themes, and as such you will not be aware of how ONE theme can make changes to the way a Course can look.
The beauty of this is that Moodle themes are versatile and can style every course differently if need be, but it would make sense too to use a Course theme, which is, I imagine your aim? However, that said, there are limitations with Course themes since they can be overridden in certain areas, which in some cases can spoil the look a Teacher wants for their course/s.
As for making your own child themes, this can be made easier by making a minimalist child theme, which just has the basics as all the code is in the Parent theme, which in your case would be Adaptable, but all the changes would be in the child theme. These kind of themes are very small and just take from the parent the necessary things like settings and some stylesheets.
Depending on the look you have in mind can you describe further what you want to change in Adaptable in those two courses? Are you talking about colour or a new look to the course page altogether?
Just curious,
Thanks
Mary