One question that should settle things: Are the marks shown in the pic dirty grease from the chain? If so, then it must be something like chain suck (although it would be one helluva coincidence this happens on both bikes at once) or an extreme form of chain slap, but that's still bottom on my list. Chainsuck could happen when running cross-chained (small-small). That's bad for the drive train. I hope you don't do this?