Hi ,

you can overwrite methode create and copy of stock.move

class stock_move(osv.osv):
    _inherit = 'stock.move'    
    _columns = {          
               'qty1': fields.float('Quantity 1 ', 
                digits_compute=dp.get_precision('Product Unit of Measure'),  
                'qty2': fields.float('Quantity 2', 
                 digits_compute=dp.get_precision('Product Unit of Measure'),                  
                 }   

    def create(self, cr, user, vals, context=None):
         product_qty=vals.get('qty1')+vals.get('qty2')
         vals['product_qty']=product_qty 
         return super(stock_move, self).create(cr, user, vals, context)

stock_move()

Thanks.