To resolve the actual issue, it was not necessary to transfer the products from warehouse to another.

I solved the problem by creating third warehouse (WH Main), assigning one location to it (WH Main/storage) and then assigining locations of WH1 & WH2 to use WH Main/storage as their parent location. 

This solution was actually found from the Odoo docs here: https://www.odoo.com/documentation/15.0/applications/inventory_and_mrp/inventory/routes/concepts/stock_warehouses.html