If you're playing two balls, and the first goes in the water, is the second one you hit supposed to be your third stroke for the first ball, or you first stroke for your second ball? I bet you'd chose the latter if you holed out. Fortunately for anyone justifying a hole in one when playing two balls, there's not really rules behind playing two balls, because it's not an official round, and none of your scores would really be official.
I'd be frustrated had it happened to me. I prefer to play by the rules to the best of my knowledge and ability as much as I can so I can honestly post a good round, or if by chance it happened, a really rare and great shot.
The practice round thing doesn't really apply.. It'd be an ace if you played the same course twice in a day, sure, unless there's a rule about that? But I'm guessing there's not, since tournaments do it under certain circumstances plenty.
I personally think it's hard to argue.. I mean, the argument is stronger if it's your first shot, sure, but I don't see how it'd be justified on your second shot, or tee shot in the scenario of a par 3, as you literally would've had a practice shot already (which I understand hardly increases your odds of making an ace), but since the argument for the latter is much stronger, by default, it carries over to the first scenario, and it just doesn't count when you're playing more than one ball, and neither does your score. If your score for an entire round doesn't count for anything, how does your score on one of the holes count for anything?
Not sure the point or if you answered the question. I told the OP it was not a HIO as it was not his first shot playing that hole as he played holes that round (similar to me putting it in the water).
Now if he had played multiple balls on I don't care how many holes and walked up to this tee box and got a HIO on his first ball, it is a HIO.
I was only using an example.
Here is a situation I had. Junior were teeing off so I decided to run down 18 and played 17 (par 3) & 18 while I waited as could see they were empty). Proceeded to then follow the Juniors. They were slow as mollasses and again then whil waiting went to #15 (another par 3 and hit a few tee shots) and played the holes. Finally they were gone after 9.
Now if I got a HIO when I played 15 or 17 while waiting on the juniors or when I actually got around to it it is in my opinion a HIO as long as the first hit whenever I play the hole.