symfony2 form, many fields for the same property

Question

I need some help. I have 3 entity with relations User m2m Role, RoleGroup o2m Role and need to create form with select multiple tag with roles for each group, some as:

RoleGroup1
role_1_in_group_1
role_2_in_group_1
role_3_in_group_1

RoleGroup2
role_1_in_group_2
role_2_in_group_2
role_3_in_group_2

How I can create many fields for one property? Any ideas?

For example

$builder->add('roles_group_1', 'entity', array(
        'class' => 'OrfosCoreBundle:Role',
        'multiple' => true,
        'property' => 'name',
        'query_builder' => function(EntityRepository $er) {
            return $er->createQueryBuilder('r')
                            ->where('r.group = 1');
        };

$builder->add('roles_group_2', 'entity', array(
        'class' => 'OrfosCoreBundle:Role',
        'multiple' => true,
        'property' => 'name',
        'query_builder' => function(EntityRepository $er) {
            return $er->createQueryBuilder('r')
                            ->where('r.group = 2');
        };