[Django]-Django template if or statement

and has higher precedence than or, so you can just write the decomposed version:

{% if user.username in req.accepted.all and user.username not in req.declined.all or
      user.username not in req.accepted.all and user.username in req.declined.all %}

For efficiency, using with to skip reevaluating the querysets:

{% with accepted=req.accepted.all declined=req.declined.all username=user.username %}
    {% if username in accepted and username not in declined or
          username not in accepted and username in declined %}
    ...
{% endif %}
{% endwith %}