I have not done this, but below are some resources for you. Based on what I’m reading you can currently do this using an XML EPG feed if you are willing to put in some extra work, and there is also a thread requesting that Plex add the functionality natively.
- This thread is a request to add functionality to allow multiple EPG sources - you may want to vote for it.
- This is an overview of using an XML based EPG source
- And this thread discusses how to combine multiple XML EPG sources
I’m not able to help with any problems or questions, but hopefully this puts you on a path to do what you need…
