Hi,

You can define the second field in which the value of the first field has to store as a Related field.

first_field = fields.Many2one('res.partner', string="Field 1")

second_field = fields.Char(string="Field 2", related=first_field.name)

Check whether the above method is working or not. If not write value to second field using the onchange.

@api.onchange('first_field')

def set_second_field(self):

      if self.first_field:

           self.second_field = self.first_field.name                                                                                                                 

Thanks