I hope you're doing well today. Thanks for the question around IdPs for Private and Audience Specific Pages.
It is only possible to use one IdP for a setup for authentication on these types of pages. The way it currently works for us only allows this one setup. If you do have multiple IdPs to use, they would either need to be consolidated on one or separated by page since each page can have its own setup.
Hopefully that helps answer the question. I hope you have a wonderful day!