Usually, merchants that own a few stores, have multiple Facebook pages as well. Each page might belong to a different store, while they all belong to the same Facebook account.
To successfully connect the relevant pages to the relevant stores, please follow the following steps
Go to the Settings tab and click "Add Account" or "Change Pages" in the Facebook section.
Select all of the pages that you're using across all of your Shopify stores
The reason you need to select all of the pages and not just the pages that are relevant to your current store is as following: Facebook sees Post Studio as one app. It means that if you choose to connect one page only, all of your stores will have access to only the page you've selected and will lose the permission for the other pages associated with your account. You should select all of them, and in the next step, we'll filter out only the relevant pages
Grant all permissions and click Done.
Now you can select only the pages that are relevant to the specific store you're in right now
You should repeat the same steps in your second store.