With the new wow pets, they say you can only redeem it to one warcraft license on a battle.net account. However, I was successfully able to redeem the code two two accounts.
No effective way to screenshot this of course, but there you go, might as well try it if you're going to get the pet anyway.
1. Buy the pet
2. After confirmation you'll be presented with a page that lets you redeem/gift the pet.
3. Choose to redeem the pet, then choose the license you want to have it on. Don't close the window.
4. Repeat step 3 for every license you want the pet on.
No guarantees or warranties on how long this will work or whether or not they'll take the pet away at some future time if you do this. But for now at least, it is working.