Okay so it seems extremely obvious that the two people are one. The clues are everywhere.

-Anthony is a failed actor, he has 3 acting credits, the most recent being 2005, yet he has a nice apartment and a motorcycle.
-When Adam visits his mother she comments on his nice apartment (opening scene she leaves a message berating him for his messy/poor apartment, presumably the one Adam lives in) and tells him to give up his dream of being an actor.
-After Helen sees Adam for the first time Anthony berates her for forgetting to get him blueberries, and when Adam is visiting his mother she offers him blueberries, he responds that he doesn't like blueberries, to which she replies "of course you do".
-When Adam first rents the movie he sees Anthony in, the song "The Cheater" is playing, with the lyrics "look out for the cheater" being sung.
-Both struggle with insatiable urges for women. Anthony has cheated before ("are you seeing her again?"), and when Adam is meeting with Anthony for the first time he can't help but look longingly after the woman walking in front of him in the hotel hallway.
-They have the same scar on their stomach
-Adam's apartment has basically nothing in it. Boxes, a bed, and a couch. There are no pictures or anything on the wall. It looks exactly like that of a 2nd apartment for affairs or one recently moved into after being kicked out.

It also seems clear Anthony is the dominant personality and the "real" one. He created Adam as an escape from his life in which he feels trapped (more on that later).

Okay onto the spiders. The spiders are a metaphor for the way Anthony/Adam perceive the women in his life. He feels trapped in their web. Female spiders have even been known to kill their mates after, or even sometimes during, copulation. This symbolism is overwhelming throughout the movie.

-Opening scene at the sex club, Anthony (and other married men) are aroused at the woman stepping on a spider.
-The wires above the roads in the city they live in resemble a spider web
-In Adam's lecture he teaches about dictatorships and says, "control, it's all about control..."
-After visiting his overbearing mother the very next shot is one of the entire city, and a mother spider standing above it
-When Anthony & Mary crash in the climax of the movie, the broken windshield closely resembles a spider web. Before they crashed Mary berated Anthony for "not being a man".

This brings us to the end. While very surprising, even scary, the first time you see it, this also seems very straightforward. The spider, Helen, despite being much bigger than Anthony, backs against the wall in terror. Why? Because Adam/Anthony had just shed the Anthony personality, the one that felt trapped and felt the need to stray. He and Helen had an amazing night together where she said, "I want you to stay" (talking to the Adam personality). But just when things seem good, Anthony discovers the envelope in his jacket with a new key to the sex club and tells Helen he has somewhere to be tonight. So Helen's fear is due to the re-emergence of the part of Anthony's personality that wants to escape. Once again, there are clues to this.

-During Adam's first teaching lecture, he says, "It's important to remember this, this is a pattern that repeats itself..."
-He also says, "All the greatest world events happen twice. The first time it was a tragedy, the second time it was a farce". So Adam/Anthony has accepted that history is destined to repeat itself.
-To complete that, the movie opens with Adam's mother leaving a voicemail on his machine. The last thing Helen says is "your mother called". It's a loop.

So here are a few of my questions:

1) What's the deal with the picture? When Adam first sees Anthony in the movie, he digs a picture out of one of his boxes. The picture is cut in half and only his himself. Later in the movie, Adam once again sees the picture at Anthony/Helen's apartment. But this time it's the entire picture. The torn part is Helen. I feel like Occam's razor---two of the same picture---would be kind of lame and a cop-out. Part of me thinks this means that the story of the movie is being told non-linearly.

2) This brings us to Mary. Was she the one Helen had caught Anthony with prior? Did they get caught because of that car accident? This makes sense to me because of the way that sequence of events is edited. They intertwine Anthony & Mary having sex (Mary discovering the wedding ring tan on his finger) with Adam in bed with Helen. He wakes up and then cries apologetically on the couch as she consoles him. It is almost as if he is apologizing for that. The film opens with the quote "chaos is order yet undeciphered". This leads me to wonder what the true timeline of events in the movie is. I believe it all happened. It just seems likely to me that some of it (specifically the Adam/Mary relationship) did not occur in order with how we as viewers saw it.