These two Kingdoms don't get to form an empire so I think that they should be able to....call it Pollack-Hungary
I'd prefer that empires be dynamically created, rather than every stretch of land having its own empire.
I don't think empires should be put into the game just for the sake of having them there. Empires make sense in places where uniting certain kingdoms makes clear cultural or geographical sense, like Britain or Iberia or Scandinavia. But uniting Hungary and Poland does not make sense either culturally or geographically, and any empire that bound them together would be a very strange thing to justify. Uniting those kingdoms in a personal union under one dynasty is the most that you can justify. Making an empire out of them is such a specifically bizarre ahistorical ambition that it makes sense for a mod, but not for the base game.
That's the reasonable way to go. At least such empires would have an in-game reason to exist, as opposed to most of the current formable empires. I appreciate the way CK2+ handles new empires - you need four kingdom titles and the decision appears to form an empire named after your primary title. You could argue that three kingdoms could be sufficient in some cases (as Poland-Lithuania-Rus-Hungary seems a little too massive when compared to Scotland-Isle of Mann-Ireland-Wales), but the principle remains.I'd prefer that empires be dynamically created, rather than every stretch of land having its own empire.
Since you linked to the Wikipedia article, you should have realized that this »empire« comprised of Poland and Hungary was actually a personal union which lasted for a whopping seven years; the dynasty in question was of Lithuanian origin, and the king Ulászló the First was actually Polish. Also, the name »Etelköz« is totally inappropriate since it usually denotes the Magyar urheimat; that's like renaming in-game Taurica »Old Great Blgarija« and giving Bulgaria de-jure claims on it (and then give them de-jure claims on everything between Constantinople and Kiev for good measure).
Vladislaus Jagiello whom several history books call Vladisla(u)s II. After being elected and crowned King of Hungary, Vladislaus moved his court to Hungary from were he ruled both countries and his children were born and raised.