Technically, training an animal to attack anything takes twice as long, but that trick is still only one trick...
The attack trick says that
Teaching an animal to attack all creatures (including such unnatural creatures as undead and aberrations) counts as two tricks.
Read that carefully! Only for teaching the animal the—let's call it—attack anything trick does the attack anything trick count as two tricks. For all other purposes (e.g. computing how many tricks an animal can have, how many tricks an appropriate magic item can grant) the attack anything trick is but one trick. To be clear, the skill Handle Animal on Common Tricks says
The following tricks can be taught to animals by training the animal for a week and making a successful Handle Animal skill check against the listed DC.
Because the attack anything trick counts as two tricks while the handler's teaching it, the handler takes 1 week and a successful Handle Animal skill check (DC 20) then another week and a successful Handle Animal skill check (DC 20). If both skill checks are successful, the handler's taught the animal the attack anything trick. For all other purposes, the attack anything trick counts as one trick.
"That sounds a little shady..."
While this really is what the rules say, I, too, for example, have just written down on my character sheet that my inquisitor's attack bird had the attack trick twice and counted it as two of the bird's tricks. Things are easier that way, and nobody'll fight you if you do that. But doing that causes some problems.
The game has no mechanism for adding the attack anything trick to the animal's already learned attack trick. Further, once an animal's learned a trick there's no readily available way to cause the animal to unlearn that trick.
So while it seems shady to have the attack anything trick be taught as two tricks but learned as one trick, under this technical reading, an animal that's already learned the attack trick can learn the attack anything trick and those tricks together occupy two tricks the animal can learn. But were the GM to mandate that the attack anything trick counts as two tricks the animal's learned (rather than two tricks the handler's teaching), an animal that already know the attack trick would be forced to spend three of its precious tricks to do the same, one on the attack trick and two more on the attack anything trick.
...But you've proposed reasonable house rules
What would be awesome and reasonable is if the attack trick said that a trainer could teach an animal that's already learned the attack trick the attack anything trick as a lone trick, perhaps taking twice as long to teach. I mean, really, that's how it should work. Unfortunately, the game doesn't say that, but the GM could rule that's how it works.
So, assuming the DM instead counts the attack anything trick as a different but improved version of the attack trick counting as its own lone trick (perhaps treating the attack trick similar to a feat's prerequisite for the attack anything trick), adding only that second trick after an animal's learned the attack trick is a reasonable house rule no matter how that second attack trick's added (e.g. magic items, teaching).