As you know Facebook business manager is connected with the website. In your case you have two website and you have one business manager with two pages & it is not possible to connect the two websites to one business account. So it will be good you can create a new business manager from the second website address. And remove the second page from the first business manager and connect it with the new business manager that you create And after that you are able to connect your second page with the second website.
Note:- You can only create 2 business manager from one account. Thanks
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