It seems like it should be possible. But I can't see how to do it.
Is it possible to add an additional site page library in a SharePoint site.
I can add document libraries just fine but it doesn't appear as I can create a page library.
The Site Pages library is a special library type in modern sites. It cannot be duplicated at this time. The most common reason for wanting another Site Pages library is usually for permission control en masse rather than on each individual page. The current recommended way to handle this is to use folders in the library instead.
By default, folders are disabled for the Site Pages library, but it can be enabled from the library's settings page under the Advanced Settings section.