Mentioned this on discord, but: Multiple collision masks/hitboxes are already possible in GDevelop. You just need to click on the bottom right most + when editing hitboxes. It’ll add a new polygon for you.
You cannot target hitboxes separately though, they’re all treated as part of the object’s collision.
If you want to target them separately right now you’d need hidden objects used for hitboxes with unique names.
Separately, this feature request currently already exists here: Multiple mask that can be used individually - #7 by Midhil