[Answer]-Forms that comprise of two models

Use get_or_create on the User model to fetch a user if one with that email already exists:

Here is one way you would wire this up in your view:

def some_view(request):
    form = SomeForm(request.POST or None)
    if form.is_valid():
        obj, created = User.objects.get_or_create(email=form.cleaned_data['email'], 
                                                  first_name=form.cleaned_data['first_name'],
                                                  last_name=form.cleaned_data['last_name'])
        ContactRequest.objects.create(user=obj,
                                      message=form.cleaned_data['message'],
                                      datetime_created=datetime.datetime.now())
        return redirect('/thanks')
     return render(request, 'form.html', {'form': form})

You can help yourself a bit by setting the auto_now_add option for the datetime_created field; this way your timestamp is automatically saved.