For anyone working on this like I was, there is "sort" of a bug with this, or at the very least, an unintentional interaction.
If you have "Speak with Animals" active during the cutscene at night with the two of them, you will not pop the achievement. Playing out the cutscene both with and without Speak with Animals produces several different options. The achievement only works in the version where you can't understand what they are saying.
Also, you don't actually pet them both "at the same time" in this version. You pet one, then the other, and the achievement pops. Funny enough, if you play the scene out being able to understand them, you do actually pet them "at the same time", but no achievement.