If you define a "app" and use it inside of your "Django Project" I assume you use INSTALLED_APP's in settings.py to make it known within your environment.
If you look it from this point - its the same as if you use "django-social-auth" in two different projects/services and you share the same DB. I can't judge if its common or uncommon to use the same DB, its a design decision you have to make and you need to be happy with.
If you just like to have users and webusers seperated in your admin, please have a look at separating-staff-and-user-accounts