I ran into the 'already applied' issue on my 4-subscription account the first time I tried to apply the codes, but when I tried a second time a day later, the option to select an account for each code was available - and the account that already had one of the mounts awarded showed properly as already having that benefit applied. My _guess_ is that it is an intermittent error in the billing system - sometimes you get through fine, sometimes you don't.
Edit: Another point for speculation - the first attempt (that had the 'already applied' error) was mid-morning, the second (and successful) attempt was in the very early morning (~3AM EDT), so load on the billing system might play a factor.
Last edited by Boskone08; Mar 19 2013 at 02:54 PM.
Be well and good questing, all! See you about Middle Earth another time!