I always thought Portuñol/Portunhol referred to any mix of Spanish and Portuguese.
I use it myself when talking to Portuguese speakers who don't have English.
Basically I just speak Spanish except:
- where I happen to know there is a different word in Portuguese (like hablar vs falar)
- where I know there's a different pronunciation of a same or similar word (like me vs meu)
- where I happen to know there's a different grammatical structure (like gustar vs gostar)
- where I know there are equivalent endings for latinate words (like conversación vs conversação)
Of course you can only avoid any embarassing pitfalls like false friends if you know about them. But of course the person being addressed also adjusts how they're listening to allow for such things.
And of course sometimes this isn't enough and you have to hunt for synonyms or circumlocutions, or just give up, smile, and shrug your shoulders (-:
Basically it's not much different to Spanglish except that due to the similarity of the languages the intelligibility and usefulness is orders of magnitude greater.
So basically per my understanding everybody will do this according to their level of understanding of the other language, meaning everybody will speak their own version. (Oh and of course Portuguese speakers will do the opposite.)
If there actually is some official, standard, or formalized Portuñol/Portunhol, I'd love to hear about it!
Thanks to Richard's response on the meta post about whether Portuñol and related topics are within our scope I have learned that there is indeed another Portuñol which actually is its own language, though doesn't have an ISO language code and I assume it hasn't been codified into a standard, but must be the native language of a community. You can read about it in its own Wikipedia article, Riverense Portuñol language.