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Actually there was a similar idea to that used in Phantasy Star 2. Nei First and Nei.
Nei First was a biological experiment combining human DNA with various and unspecified DNA types. She was deemed a failure and fled before she was to be killed. Nei First became quite evil and insane plotting revenge for being deemed a failure and the attempted killing. Part of her disagreed with these thoughts and possibly due to instabilities in the genetic structure that part split off. This was Nei. Nei eventually had to fight Neifirst and no matter what she would die in this fight because if Nei First were to die she would as well because Nei First was the original and had the most of their spirit.
Now for the Dante/Virgil thing to work there are a few alterations to the story that would need to be done to have an acurate split being idea, and it would have to follow a similar method to the Nei/Nei First thing. That would mean ALOT of Dante's past poping up in the new game.
First of all Dante would need the larger portion of the soul to avoid dieing when Virgil dies. Second the cause for Virgil would have had to happen pre-birth most likely to give the idea that they were twins, perhaps some meddling was done by an evil spirit. And lastly something or someone important would have to bring this information to the surface.

So while it is possible that they are not brothers but one split being, it is highly unlikely as you said.

O_O Now that Im done rambling I may go to bed. whee

I agree with pretty much everyone who has posted before me. The idea is a possibility, but I doubt it's true in the game. However, I have a question about your "theory": So if Dante and Vergil are part of the same soul, are they separate entities that are essentially one soul? Or is it really just one entity that possesses an internal struggle within himself, and is the only one that sees Vergil (assuming he's the second entity) in the game? In other words, are Vergil, and the other part of the pendant, and his memories, merely the works of his imagination?

These are just some things that I wasn't sure about.

i think that they are. if there is some speculation of this then hopefully it will be answered the DMC3

BTW- I commend you, for actually stopping to think. alot of people, would never think enough to come up with something like that. ::gives you a cookie:: xd

Well, it's possible, but I doubt it though.

Yeah, we know that for a fact. But there could have been a pretty cool thingy if Capcom got off their asses and got all... creative.

Thier twin brothers.

After vergil "killed" dante, he slicked his hair back and that gave him the new look. Before that he looked just like dante except with blue eyes.