onchange is normaly used for changes in UI. Rather use depends and make the field compute.

x_totalunite = fields.Float(compute='_get_value', store=True)

@api.depends('product_uom_qty', 'x_unitparcarton')

@api.multi

def _get_value(self):

   for rec in self:

      if rec.product_uom_qty and rec.x_unitparcarton:

          rec.x_totlaunite = rec.product_uom_qty * rec.x_unitparcartron                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    

I hope the fields are in the same model else it can't work. Then you'll need to use related fields.

Try like this,
@api.v8 
@api.onchange('product_uom_qty', 'x_unitparcarton')
def onchange_field(self):
if self.product_uom_qty or self.x_unitparcarton:
    self.x_totalunite = self.product_uom_qty * self.x_unitparcarton