You can install another web adaptor and leave the authentication at Anonymous and then make sure that "Allow anonymous access to your portal" is enabled. Please note that the recommendation is to disable anonymous access.
I apologize for the misleading information, but after a bit more research and discussion, this is NOT possible. In order to register two different web adaptors with Portal, you need to set the Web Context URL:
This defines an entry point into the Portal, and in the case of multiple web adaptors, it's meant to be a load balancer that can balance requests to the web adapters. Setting the Web Context URL also disables this error from coming up, so you can actually register multiple different web adaptors to Portal, even if you don't intend on using the web context URL to balance requests to them. This may make it seems like you can have multiple web adaptors with different security settings, but that's not the case. To put this in perspective, let's say you have a Portal you want configured with IWA, and you set the Web Context URL to point to a reverse proxy that then points to a web adaptor that is open, for example https://public_portal.domain.com/open. This allows anonymous access. You have a separate web adaptor configured with IWA that's accessible through your internal network, so domain users can sign in and create content, accessible through https://internal_portal.domain.com/iwa. Since only named users create content, all items are created with the URL set to https://internal_portal.domain.com/iwa. When named users create content, the URL for those items are going to point to https://internal_portal.domain.com/iwa, so when external users reach the portal through https://public_portal.domain.com/open, all items will reference https://internal_portal.domain.com/iwa, and they won't be able to reach the item as they can't be authenticated correctly. We are going to update the documentation to explain this further. I'm also interested in how Adam from that other post configured his Portal.