Hmmm. Dawn and Buffy from BtVS, and what about Marissa and Kaitlin? They are at least as close as Kirsten and Hailey. And the annoying sisters from Charmed.

Ooooh! Veronica Mars! Duncan and Lily, Dick and Beaver. In fact, Ryan somehow in Neptune (either as a Kane or a Casablancas) sounds like it would be fun to write. I'd be up for that.