class BusinessProfileForm(forms.ModelForm):
  class Meta:
    model = StUserDetails
    fields = ('user_address1', 'user_address2', 'user_city', 'user_state', 'user_country','user_zip',)

class AdditionalDetailsForm(forms.ModelForm):
  class Meta:
    model = StUserAdditionalDetails
    fields = ('phone_number', 'fax_number', 'corporation_number', 'tax_number', 'custom_name1','cutom_field_value1', 'custom_name2','cutom_field_value2')

class UserProfileView(LoginRequiredMixin, FormView):
 template_name = 'users/add_user_details.html'
 success_url = '/dashboard/'
 form_class =
   [
      BusinessProfileForm,
     #  AdditionalDetailsForm,
 ]

 def form_valid(self, form):
     if form.is_valid():

         profile = form.save(commit=False)
         profile.user = self.request.user
         profile.save()

         messages.add_message(self.request, messages.SUCCESS, _('Profile data has been successfully updated.'))

         return super(UserProfileView, self).form_valid(form)

<form action="POST">
 <div>BusinessProfileForm(form1) <div>
 <div>AdditionalDetailsForm(form2) <div>
  <button type="submit">Save</button>
 </form>