This is my code: 

class tickets(osv.Model):    
    _name = 'tickets'   
    _rec_name = 'code'    
    _columns = {        
        'code': fields.char(string='Reference'),        
        'title': fields.char(string='Title', required=True),        
        'description': fields.text(string='Description', required=True),        
        'status': fields.selection(            
            [('draft', 'New'),             
            ('open', 'In Progress'),             
            ('pending', 'Pending'),             
            ('validate', 'To Validate'),             
            ('done', 'Closed'),             
            ('cancel', 'Cancelled')], string='Status', required=True),        
        'related_ticket_ids': fields.many2many('tickets', 'related_tickets_rel', 'ticket_id1', 'ticket_id2', string='Related tickets'), 
    }

Imagine that we have 3 tickets created: ticket1, ticket2 and ticket3. 
What I want is when I add in ticket1 (in the 'related_ticket_ids' field) the ticket2, in the ticket2's 'related_ticket_ids' field appears ticket1.