The current specification allows the portaled page to optionally "adopt its predecessor", putting it into a portal.

It's not clear whether this functionality will fully survive, as we refocus portals around a more tightly scoped prerendering-focused MVP. For example predecessor adoption might only happen automatically (via the browser's already-existing back-forward cache), instead of via an explicit JS API. We'll see.

Multiple portals are generally like multiple iframes, although we may also impose some restrictions on "backgrounded" portals (e.g. the automatic predecessor-adoption-into-the-bfcache mentioned above).