I realize up front this post won't be a lot of help, but I can pass along what I've heard...
We have a split stay coming up in Oct - Pop (testing) first and SF (non-testing) following. I have been able to make FP+ reservations through the entire trip and everything (tickets, resort reservations, FP+) shows up in MDE.
I called and asked the IT department whether or not I would be able to link my room key at the GF to the Magic Band. He said - absolutely... once you have the magic band it is yours to keep and all room keys, charging privileges, tickets, etc. will be addable for any future stays - including the GF as a non-testing resort.
Fast forward to yesterday when I was dealing with an unrelated MDE issue and I called Disney IT again. This time I was told "There is absolutely no way to link your GF room key to my magic bands since they are not a testing resort. If you show it to the front desk, they will look at you like you are crazy - they will have never seen one since before". I swear that was nearly word for word. I find it impossible to believe they would have never seen a band before, but I'm not sure about the inability to link them. I explained that was in direct contradiction to what a colleague of hers told me a week prior, but she insisted she was correct. The second half of her comment certainly dinged her credibility with me, but I can't be certain she is wrong across the board.
So, that leaves me with exactly 50% of the Disney employees I've talked to gave me the answer I wanted to hear and 50% gave me the answer I did not want to hear. Good stuff!
Sorry I couldn't be of more help. By the time I know for sure, it will probalby be common knowledge