i would suggest against it as knifes are known to be teritorial against ea other. but everyone experiences are different you might be able to pull it off if you get two knifes that likes each other. the chances are slim but it can work
I've had this mix for a bit. I ended up moving the BGK to the bigger tank because it was getting big, lol. While they were together there wasn't any issues. As long as you have plenty of hiding spaces, it shouldn't be a problem.
I love MFK, every time I have a question I turn here and 5 minutes later I have an answer. I too was wondering about keeping a black ghost with my royal.