Better do a little more detailed explanation, before we're accused of 'instructing with instructor in the back seat'.
As an ACP program student you may (or may not) be paired up with another student for your program.
If you are, all your instruction is still done one on one with your instructor. Your partner may, from time to time, back seat to observe and learn that way, but mostly you fly with your instructor only. This is for the preparation of your ME Private addon, instrument ticket, commercial tickets, as well as the addons for your instructor ticket. This also includes several cross countries in preparation for both your instrument check ride and commercial checkride.
Once you are done with your ME Private pilot addon and your instrument rating you and your partner (Or some other ACP pilot if you're going through the program alone) have a phase that lasts for about 3 weeks where you fly a lot of X-C flying.
The two of you fly alone, there is no instructor aboard, there is no instruction involved.
This phase is used for two important things; First and foremost to build experience. You have no idea how much experience you really gain from this phase before you've been through it. Dinking around at your local airports is one thing, but flying half way across the nation, in all sorts of weather builds experience you can't get locally.
The second purpose of this phase is to build time so you'll be eligible for your commercial rating.
So in short, aside from this 3 week portion where you and your partner fly cross countries by yourselves, all flying is more or less done one on one with your instructor.
Perhaps this'll clear up a little.