This has never been officially tested. Somethings may not work properly or at all.
For example, we store the URL to portal in the GTREP. If you have multiple instances, then selecting from this table will potentially return too many results and cause issues with the components that are attempting to get the portal URL.
For example:
When datamaker attempt to run an enterprise publish, in a best case scenario all jobs will be sent to the first portal server.
I would recommend opening an idea with some additional details regarding how you would like to utilize 2 portal instances on a single repo.