This scene truly marks 'The End' of Hedy's happiness. Hedy turns out to be a sociopath who hasn't forgiven herself for her twin sister's accidental death, and when her bond with Allie is threatened when Steven Weber reenters the picture, she resorts to murder to keep Allie to herself. It's strange to see a film made in the 1990s keep 1950s sensibilities - Hedy must be punished for her love for Allie - and uses violence in place of sex much like Strangers On A Train did. Hedy and Allie have a knock down drag out fight in the last act with lots of head banging and stabbing.
The movie ends with Allie killing Hedy, and moving on with her life.
I suppose this end shot of a photo of the two actress' combined is meant to imply they're the same person - but the movie never really supported this claim. It followed the mainstream studio idea of 'bitches are crazy' more than offering any psychological insight. Hitchcock's POV shot of Farley Granger punching out Robert Walker at the dinner party in Strangers On A Train is a much better execution of this idea.It's a very Patricia Highsmith like story, and I'm curious to read the novel by John Lutz to see if there were any major changes for the movie. In any case, it's an example of my favourite kind of romance - obsessive love gone wrong