How to delete second admin account from my MacBook

I have two users with admin permissions on my Mac running Yosemite. The problem is left over from when I used Migration Assistant to copy the contents of my old PC onto my Mac. I've tried multiple times to go in through Users & Groups Preferences, but it is not possible to remove either account when logged in under the other admin's credentials. If I'm logged in as Admin A, then Admin B is grayed out and cannot be removed ... and vice versa. How can I delete one of these accounts? Alternatively, how can I remove the admin designation from one of them?