We are facing an issue when we have 2 Edge web adapter with IE compatibility pages in a solution. The controls of one of them do not match. I tried to follow the documentation and kept the edge configuration properties same for both the adapters. I am using 19.1.92 version.
More details are mentioned in the attached file "MultipleEdgeAdapterIssue.docx".
Without getting more hands-on to test your adapters though, I don't really see anything here that points to a cause. As you've said, the documentation seems to indicate that Runtime does allow for multiple adapters assuming you set both adapters to the same properties under the Edge Configuration property. It does indicate that you cannot interrogate two at a time though. While I can't find the exact documentation on this, I suspect this means that you can't run two at the same time. I know you can create a Web Adapter and set it to MonitorAll and interrogate both pages in one adapter. That might be the solution, however I would open a support request to check first as they'd be able to update the documentation and/or assist you with your specific issue.