actually had you paid more attention you would realize that this is a spin off of sort of Romeo and Juliet
They were dead - it was an only way to escape. Blonde one was mortal injured, I guess Black one killed herself after Blonde was dead. It wasn't showen directly but if you look deeper in last part you could see this tragic end. Happy forever - after death they stay with each other because they marryed - it was an idea of their "happy ending".
Oh! And light door - it was a light of the end of tunnel - another symbol of death.
What happy ending????? This wasn't a happy ending. I wanted to know what happened to them!!! Where they live???? And how they can afford to have food, and money, and home to live!!! It will better if they found a new clan and rip the older clans!!