I have done it like that:

class MusicSong(models.Model):
    _name = 'music.song'

    related_ids = fields.Many2many('music.song','music_song_related','music_song_ids','music_song_related_ids')
    related_reverse_ids = fields.Many2many('music.song','music_song_related','music_song_related_ids','music_song_ids')
    related_songs_computed = fields.Many2many('music.song','music_song_related',string="Related Songs",compute='_compute_related_songs', inverse='_inverse_related_songs')

    def _compute_related_songs(self):
        song_ids = self.related_ids.ids + self.related_reverse_ids.ids
        for song in self:
            song.related_songs_computed = song_ids

     def _inverse_related_songs(self):
        for song in self:
            song.related_ids = [(6,0,song.related_songs_computed.ids)]

It works but I'm sure there is more correct way of doing this. I would appreciate any input.

Hello Mageno,

In odoo, you can find same functionality under Customer(res.partner).

Here Company type partner have more then one Child.

You can get some help from there.

Hope this will help you,