Hi Motasem Migdadi

This is working as designed.

• If the 2nd Semi-Finished is set as a Byproduct, then the system expect you to provide the unit cost. The actual cost calculation will only apply on the released product(s).
  This is a product limit which is a real pain when addressing any industry with a cutting process. Custom development can be made to work around this limit but it is often quite expensive.
• If you set both semi-finished products are released products (multi-products WO), then the cost would be defined based on the quantity produced. So if you managed your products in unit (Stock unit = UN), then the cost will be the same for each unit produced. If you managed your products in Kilo or square meter (Stock unit = KG or M2), then the quantity produced will be different for each format and the cost per product will be more accurate. However, you would need then to use packing unit to identify the actual number of units you have in stock.
  The downside of this solution, is the creation of multi-products work orders is manual only. again, you would need some customization budget to automate the creation of a multi-products WOs from the MRP.

You can vote for the following suggestion: www.sagecity.com/.../by-product-valuation-method-joint-cost-split-cost