However, to offer something of more substance... High Tension was good until the last 20 minutes. The revelation the the killer was hugely predictable. Even crazy people don't watch from the closet while they kill a bitch. Or simultaneously drive two vehicles. Our chase themselves through the forest and set traps for themselves and attack themselves with wide swings wielding large pieces of wood. Just because you're crazy, doesn't mean that you can defy the laws of physics.
alien, your interpretation would be valid if the movie had decided to only allow us to see the alter ego from Aja's perspective. But alas, what ruined the movie was the fact that even though it was supposed to be the same person, we as the audience were shown the alter ego unattached to the body it was supposed to inhabit.
It was a decent movie, better than most of the crap that Hollywood comes out with, but would have been much more satisfying if A) they had done a better job of dissociating the alter egos from one another, or 2) if the killer had actually just turned out to be another person.
And yes, I'm aware that I numbered off with A and 2. What you gonna do about it?