When last we left Petey he was about to step into his 3rd grade classroom for the first time again (again because, of course, he's been stuck in 3rd grade since 2004, the physics of comic strip time dilation/expansion being what they are; but really, no wonder he's neurotic). I think we all know the staggering discomfort of that first entrance into a new classroom. It's the stuff of nightmares, if the stuff is made of your everyday cringe-inducing awkwardnesses. As Petey is so finely calibrated for these things of course his psyche's going to overreact and split in two. I just like the idea that the out-of-body Petey is the aggressive one.
The first time Petey multiplied was in the strip below from late 2005 (I think). Having been a soccer parent, and a pretty lousy one, I can attest to the part about mom not paying attention.