Yes, this is possible as long as the second level nested object has a role to stop infinite recursion from occurring. Cycles are not automatically detected.

    class BaseMapper(Mapper):

        __type__ = TestType

        score = Integer()
        nest = Nested('NestedMapper')

        __roles__ = {'nested': blacklist('nest')}

    class NestedMapper(Mapper):

        __type__ = TestType

        back = Nested('BaseMapper', role='nested')
        name = String()

    obj2 = TestType(name='test')
    obj = TestType(score=5, nest=obj2)
    obj2.back = obj

    >> BaseMapper(obj=obj).serialize()
    {'nest': {'back': {'score': 5}, 'name': 'test'}, 'score': 5}