Is it possible to copy custom devices between two iPads when sharing a show?
No it is not possible.
Otherwise you can share with the community a device (by swiping left). It is not instantaneous because it needs a validation by Photon moderators then the device will appear in the fixtures list for all users
I add a precision
Customs devices into a show still mapped when your export a project and open it, they still, just you don’t see into available mapping list