You are assuming that you will get the hole perfectly straight and spaced. My guess is that you will find that with a 12 inch hole, those smaller posts will be off to one side of the hole by the time you fill in the hole. I'm also going to guess that it changes with every hole. If you get one in four that are perfectly centered, I would be very surprised.
Instead of buying a second auger, spend the money on a very good, high quality clam shell digger. Even if you get the hole perfect, you might still want to clean up the bottom of the hole before putting the post in there.
Another thing to remember is that it's very hard to get the depth perfect every time. Be sure to have a few rocks available to put under the post to build up the height. I don't like to use dirt for this because it settles.
Only use one or two rocks. You never want to have a bunch of rocks at the bottom of a hole because that is where moisture will collect and remain. Lots of rocks create a cavity, storage area for water/moisture.
In every rotten fence post that I've ever come across, the rot is "ALWAYS" at ground level. pack the post tight and build up a crown or slope around the post to get water away from it. Then in a year, go back and build it up again because it will settle/wash away.
Eddie