Hi,
First, if you want to use global products, you have to enable them in store configuration on both sites: https://docs.kentico.com/display/K81/Enabling+global+E-commerce+objects
Then you have to create a global SKU on one site: https://docs.kentico.com/display/K81/Using+site+and+global+E-commerce+objects#UsingsiteandglobalE-commerceobjects-Products
After that you can access to global SKU on the another site via standalone SKUs:
https://docs.kentico.com/display/K81/Managing+stand-alone+SKUs
Best regards,
Ladislav