I searched many times and not found a solution can solve my problem. I know that ERPNext has multiple UOMs function that means between the UOMs have a relationship, but this is not I need. In my situation, I need to separate the stock quantity in different UOMs for our products. Our industry is jewellery, so we concern the stock of unit and weight. I need the secondary UOM keep the second stock only and no calculation in any case for pricing. For example, necklace is measured in unit (Secondary) normally, but it also be measured in grams for calculate the price (Primary). If we have 5 units and 10 grams on hand, and we can sell 3 units and 6.5 grams; the balance will be 2 units and 3.5 grams.
You can try creating a custom field in Item master for alternate UOM for secondary purpose. Perhaps you can then write a custom script to have the UOM conversion for item and value to be Updated accordingly in Stocks as well.