from django.db import models
from    django.contrib.auth.models      import  User

class Atc(models.Model):
    id = models.AutoField(primary_key=True)
    atc_name = models.CharField(unique=True,max_length=100)
    email = models.EmailField(null=True,max_length=50)
    phone = models.CharField(null=True,max_length=50)
    class Meta:
        db_table = u'atc'
        permissions = (("read_atc","право на чтение"),("add_atc","право на добавление"),("change_atc","право на изменение"),("delete_atc","право на удаление"),)


class Atc2Users(models.Model):
    id = models.AutoField(primary_key=True)
    atc = models.ForeignKey(Atc)
    user = models.ForeignKey(User,unique=True)
    class Meta:
        db_table = u'atc2users'
        permissions = (("read_atc2users","право на чтение"),("add_atc2users","право на добавление"),("change_atc2users","право на изменение"),("delete_atc2users","право на удаление"),)

#coding:utf-8
from    django  import  forms
from    django.forms    import  ModelForm
from    tss.journals.models     import  Atc
from    tss.journals.models     import  Atc2Users


class   A2U(ModelForm):
    atc = forms.ChoiceField(label='ЭТЦ:')
    user = forms.ChoiceField(label='Пользователь:')
    class       Meta:
        model = Atc2Users
        fields = ('atc','user')

#coding:utf-8
from    django.shortcuts        import  render_to_response
from    tss.forms.spr           import  A2U
from    django.contrib.auth.models      import  User
from    django.contrib.auth.decorators  import  login_required
from    django.contrib.auth.decorators  import  permission_required
from    tss.journals.models             import  Atc
from    tss.choices                     import  USER_LIST
from    tss.choices                     import  ATC_LIST
from    tss.journals.models             import  Atc2Users



### --- Добавление записи ---
def     add(request):

    action = 'new'

    error = ''


    ## --- Для заполнения ----
    try:
        spr = A2U(request.POST)
        user_kod = request.POST['user']
        atc_kod = request.POST['atc']
        u = User.objects.get(pk=user_kod)
        a = Atc.objects.get(pk=atc_kod)


        if spr.is_valid():
            a2u = Atc2Users()
            a2u.atc = a
            a2u.user = u
            a2u.save()
            return render_to_response('atc2users.html',{'a2u1':get_list()[0:39],'a2u2':get_list()[40:89],'error':error})

        else:
            error = 'Выбирете значение!'

    except:
        spr = A2U()

    spr.fields['user'].choices = USER_LIST(first=1)
    spr.fields['atc'].choices = ATC_LIST(first=1)

    return render_to_response('atc2useradd.html',{'spr':spr,'error':error,'action':action})

Выберите корректный вариант. 2 нет среди допустимых значений.

somefield = forms.BooleanField(required=False)

class   A2U(ModelForm):
    class Meta:
        model = Atc2Users
# и выбросьте:
spr.fields['user'].choices = USER_LIST(first=1)
spr.fields['atc'].choices = ATC_LIST(first=1)

# Обработчик
def add(request):
    action = 'new'
    # errorы опять же лучше встроенные использовать.
    error = 'выберите значение'
    spr_form = A2U()

    if request.method == 'POST':
        spr_form = A2U(request.POST)
        if spr_form.is_valid():
            spr_form.save()
            error = ''
            return render_to_response('atc2users.html',{'a2u1':get_list()[0:39],'a2u2':get_list()[40:89],'error':error})

    return render_to_response('atc2useradd.html',{'spr':spr_form,'error':error,'action':action})

somefield = forms.BooleanField(required=False)