I’m curious to know if this Legacy challenge can be created or is already exist

Of course it can be created if you want it to. Pick a challenge that intrigues you and bend whatever "rules" there are to your liking. The challenges are there to keep you entertained, so do them your way. 

Ok thank you

Seconding JesLet40​  I rarely follow challenges exactly the way they're written. I see the rules as guidelines rather than law. Sometimes the concept of a challenge is more exciting to me than how it's written out. So I make it my own. In one of the decades challenges (a 10-gen legacy) I'm currently working on I started with two YA legacy founders already married because I wanted to. The story in my head was of a young newlywed couple just starting out. Since I'm writing it as SimLit, I'm ignoring any rules that get in the way of storytelling. In another, I've added my own "rules" (and taken out others) so the legacy is more fun and interesting to me. I feel like in legacies especially, which you must play for so long, you should do what interests you to stay with it. This extends to all challenges. Do not box yourself in. Do whatever makes it fun for you.

In any decades challenge you need to bend or change the rules, as many of them are plain incorrect. LOL 

I think you might be able to take on most any challenge and add a couple, and just change the rule to say couple instead of a sim. I hope you find one you adore with your sim couple! ❤️