This reminds me of a movie called The One starring Jet Li. The main character is married and the protagonist kills his wife and he follows him into the next parallel universe to catch him. In the next universe he meets his "wife" and falls for her there. She is not the same person but he still loves her because she is who she is. This is also true for Susannah. It may be that they didn't have the same life as the previous Eddie or Jake but they are to her essentially the same people. And I found this: Stephen King hints that Oy will be found in the same universe that Susannah travels to and will be in some form of a dog with "odd, gold-ringed eyes and a bark that eerily resembles human speech."