Well it sure is an invasive and things-changing mechanic in that region. The Polish-Lithuanian Commonwealth was a unique union precisely because it connected two very different cultures into one country. They weren't two similar countries that just decided one day that they are similar enough to become as one as happened with events like German and Italian unifications. Also this just encourages Lithuanian and Polish players to simply annex each other instead of managing a union (why unite when their lands belong to your culture group). It should be possible and fairly simple to create an event in which if both countries agreed, a Polish-Lithuanian union would be born. It is unnecessary for Polish-Lithuania to tolerate both cultures (provided it's not possible to edit by event?), Polish language was pretty dominant where things mattered anyway. A simple workaround could be a new culture "Polish-Lithuanian" which would become the main culture of Poland-Lithuania in case it is united with Polish, Lithuanian and Ruthenian cultures as acceptable/tolerated.