The Mercan lawyers’ assertion that there is EU law which makes it illegal to have two permits is incorrect, at least in its most general form. They mentioned the case of a GV holder (dependent) being refused a student permit by Austria. I don’t know about Austria, but I know for a fact that the Netherlands explicitly allows students with residence permits from other EU countries to get student permits, and it works in the specific case of a student holding a Portugal GV as a dependent.
During the application process for international students, NL universities literally have a form which asks “are you a non-EU/EEA national BUT have an EU permit?” and if you say yes and upload a copy of your permit, they say, “oh that’s nice, you don’t need an MVV visa to enter NL, saves us both some trouble, and [some weeks later] here’s the confirmation letter from IND that you will receive your permit after you get here and submit biometrics.”
IND wouldn’t be doing this if there was a blanket EU law against two permits.
There may still be issues from SEF’s side, when it comes to the citizenship stage, but it’s certainly not illegal to have two permits provided you meet the conditions of both.
It works against Mercan’s interests to scare potential customers away with this, so I assume they are doing this in good faith based on one or two examples they’ve seen.