There aren't enough x reader stories with multi-paul (no yandere that I can find).
Imagine if he fell for and had a relationship with a fellow assassin. In my mind, yn doesn't have super powers, unless you count being a great observer. Yn does have limits when it comes to her kills, for example, children are a major fuck no. Yn is good with clothing and disguising themselves. Yn had a childhood similar to paul, but no siblings and raised to be an assassin. Yn asks paul questions to better understand him like with his power. If they have biological children, yn would do whatever it for their kids to have a better childhood and not be raised like weapons and no matter how much stress she feels, especially when those children have paul's power, they'd always be there for them (which would probably make paul love yn more).
Multi-paul definitely has yandere vibes in my opinion, especially when he feels understood by his Darling and seen as a living being with feelings. He hates when yn uses their looks to help with the job and would do what he can to stop that, maybe even get yn to leave the assassin work force.