I tried that in February last year. Despite beng assured by the front desk CMs that our charges would go to our separate credit cards, they all went to my friend's card. Luckily, I went to the front desk at the end of the second day, to see if it was working correctly, and found that it wasn't working. Thank goodness they were able to manually separate out our charges (it took about 20 minutes to separate them out). They assured me that it would work going forward, but when I went to the front desk the end of the third day, iall of the charges were still going to my friend's card. Took another 20 minutes to separate the charges out. At that point, we gave up and I stopped using my Magic Band to charge to the room.
It's a shame, because it was so easy to use the Magic Bands for charging! But it would have been nearly impossible to keep track of whose charges were whose; we both charged little things all day long.
So my advice is not to believe them when they tell you they can separate the charges. They'll tell you they'll be kept separate because each MB has its own individual PIN code, but chances are all of the charges will go on whatever credit card is the main card being used on the room.