Pretty much all correct comments on the difference between Biff and Riley. Biff has three tufts of hair – Riley has two. Riley has two lines down the center of his face – Biff has one. I recently changed their ears too. Biff’s ears are more triangular and has those three black curved lines on the underside. Riley now has a more square shaped ear.
I’ve often thought about and experimented with making them more visually distinct, but it’s been a slow process. I’ll probably make more changes down the road to Riley. Biff seems pretty much set to me. I think anything I can do to make it easier to tell them apart will be helpful for readers.