subjects = Table('subject', engine,
                 Column('id', Integer, primary_key=True),
                 Column('parent1', Integer, ForeignKey('subject.id')),
                 Column('parent2', Integer, ForeignKey('subject.id')),
                 )

class Subject(object):
    pass

mapper = mapper(Subject, subjects,
                properties={
    'parent1o':relation(Subject, subjects,
                        primaryjoin=(subjects.c.id==subjects.c.parent1),
                        foreignkey=subjects.c.parent1),
    'parent2o':relation(Subject, subjects,
                        primaryjoin=(subjects.c.id==subjects.c.parent2),
                        foreignkey=subjects.c.parent2),
    }
                )

Subject.mapper = mapper