Adding a ManyToManyWidget to the reverse of a ManyToManyField in the Django Admin

This is possible to do with a custom form.

from django.contrib import admin
from django import forms

from models import Post, Tag

class PostAdminForm(forms.ModelForm):
    tags = forms.ModelMultipleChoiceField(
        Tag.objects.all(),
        widget=admin.widgets.FilteredSelectMultiple('Tags', False),
        required=False,
    )

    def __init__(self, *args, **kwargs):
        super(PostAdminForm, self).__init__(*args, **kwargs)
        if self.instance.pk:
            self.initial['tags'] = self.instance.tags.values_list('pk', flat=True)

    def save(self, *args, **kwargs):
        instance = super(PostAdminForm, self).save(*args, **kwargs)
        if instance.pk:
            instance.tags.clear()
            instance.tags.add(*self.cleaned_data['tags'])
        return instance

class PostAdmin(admin.ModelAdmin):
    form = PostAdminForm

admin.site.register(Post, PostAdmin)

That False in there can be replaced with a True if you want vertically stacked widget.