Source Advanced Class Guide pg. 150
Your link with your companion creature allows you to share your magic with it.
Prerequisites: Ability to acquire an animal companion, eidolon, familiar, or special mount.
Benefit: When you are adjacent to or sharing a square with your companion creature and that companion creature has this feat, you can cast a spell on yourself and divide the duration evenly between yourself and the companion creature. You can use this feat only on spells with a duration of at least 2 rounds. For example, you could cast bull’s strength on yourself, and instead of the spell lasting 1 minute per level on yourself, it lasts 5 rounds per level on yourself and 5 rounds per level on your companion.
Once the spell is cast, you and the companion creature can move farther apart without ending the effect.
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The problem lies in the wording of the Prerequisite. It requires you to have the ability to acquire an animal companion, familiar, eidolon, or special mount. It is also a teamwork feat, which requires the person (creature) you are doing it with has it as well. Unfortunately, no companion that I know of has the ability to acquire companions of their own so they are ineligible to learn it, leaving the character with no one to share it with.
I'm sure this was written with Hunters in mind, as the animal companion doesn't need to meet the feat requirements due to Hunter Tactics, and is published in the same book as the Hunter class. But Hunter is not a requirement, and it clear opens the doors to other classes with the wording in the requirement mentioning other companions.
One of my local players texted me the this question while I happened to be at GenCon, and I happened to be sitting a few tables over from John Compton so I went over and asked him. He looked over the wording, gave me his answer, then suggested I post it to the forums so that could be officially noticed and errataed. Thus this post, hopefully this is the right place to put it.