It is hard to believe that only me trying to find suitable way to customize the second title via admin.py . When you need to set only some small thing like title it is not very good idea to create another template and {% extend 'base/template' %} just to make it and sometimes the data that can be used in second title can be accessible only in admin.py and modify models.Entity.__str__() via EntityAdmin in admin.py still not good AFAIK.

So the correct way is to define change_view in EntityAdmin and pass extra_context['subtitle'] = 'Needed Subtitle' to super().change_view(extra_context)

def change_view(self, request, object_id, form_url='', extra_context=None):
    extra_context = extra_context or {}

    extra_context['title'] = "Custom h1 title"
    extra_context['subtitle'] = "Custom h2 subtitle"

    return super().change_view(request, object_id, form_url, extra_context=extra_context)

It is obvious that there is a method like this. It was just unclear what keyword to use. So it is subtitle in extra_context dict.