I don’t think it’s FUD, so much as “possible but difficult”. Typically, the rules work out such that the requirements of one residence permit basically exclude the possibility of holding another permit. e.g. if you hold a PT D7, how are you going to stay 183 days in DE for an aufenthalt? And even if there aren’t time issues, there’s 27 different legal regimes and not all of them feel the same way, so you have to negotiate a field of landmines. Plus, will the Schengen Information System actually accept it or spit out your permit information? And do you want to keep two lawyers on-call for if something crops up?
I haven’t spent a ton of time pawing the EU legal framework, but I haven’t run across any prohibitions. That said, I also haven’t run across anything that would be particularly accommodating either.