How I can add two or more fields of related products

Hi,

There is no option to do that.
If you're a developer, you can edit the file "form" of the view "product" for your backend template in order to add extra related products fields, and add the corresponding columns in the hikashop_product table in your database, and add extra related products modules in the "modules under the product page" setting of the HikaShop configuration, and modify the file components/com_hikashop/views/product/view.html.php in order to load the related products of your extra fields for the extra related modules in the listing function.
If you're not a developer, you'll have to hire one to do that for one. Any PHP developer should be able to do that, otherwise, you can contact our partners:
www.hikashop.com/home/our-partners.html