You know, maybe I don't see something, but how are they similar? Maybe the faces - slightly, exceptionally slightly, but the haircuts, bodies, postures, and of course the general impression - two children vs two teenagers turning into young women. Could another victim be a young woman, blonde, or a redhead, slim, with a mass of hair pulled up? Yes. Two kids are two kids.
If there was similarity, then maybe the BG was searching for children, but in the absence of the opportunity, decided that two teenagers might work. And then he had to pass them and look into their faces, to see that they were childish, no makeup, etc.
Question, how common would it be for younger children of Delphi to get on the bridge unattended? It doesn't seem to be a safe place. If BG were hunting for younger kids, he'd probably look for them on the trails.
ETA: If we assume that BG killed both Abby and Libby + Lyric and Elizabeth, then this no doubt makes him a SK. Then there is no reason to believe that these were his only victim. And because SK usually would kill in own neighborhood, maybe there is a need to look at our disappearances/bodies in IN and IA, or see who migrated between these two states, maybe, IL, too?