Here's what worked for me: Ensure your that your Aulani booking is linked to the same Disney account that you use to sign-in to MDE with. You won't see the Aulani booking *in* MDE, but as long as they're both linked to the same Disney account, they are still linked behind the scenes. Ensure the magic band you want to use at Aulani is linked to the same MDE account, and the magic bands will then just work for you.
If they don't work, front desk will be able to fix things up, but all of this worked for me automatically.