If you want to split hairs: There is no law that allows lane sharing/splitting/filtering. There's only no law to prohibit it. The motorcycle handbook is weasely, implying that it's inadvisable without saying anything about what's permissible. The driver handbook does not mention it at all. This is how everyone seems to like it. We have a lot of 2-lane left turn lanes around here and it's pretty unclear what you are allowed to do. It seems like the answer is "whatever you think will work just don't piss anyone off"