Now, there is a sentence of which I think everyone should think of after reading the novel.
"Whenever you see your shadow Jamie, just remember you're looking at me."

Abby tells this sentence, when Jamie deserts Abby.
This sentence shows, that both share the same fate, which make them "transform" into shadows.
And because of this, the two "Shadows" can affiliate with eachoter, so if they do that, they grow stronger and they can solve problems together.