Parents can generate additional activation codes from their own profile. Parents can find the instructions "How do I add another parent to my child as a parent?" In their own FAQ under "General questions".
Alternatively, you can do this if the parents don't want to talk to each other. To do this, go to the user management (drop-down menu at the top right) and select the profile of the child for whom you want to add another parent.
In the profile, scroll down until you see the option "Add Parent".
If you click here, an activation code will be generated which you can give to the parent accordingly. This code is only active for 48 hours for registration. If this is not used to log in during this time, a new code would have to be generated.