Nobody can save other people from themselves. They need to do something about their loss of interest in each other. You're not the answer. If you become the answer, that just means you will end up not enjoying your life either. People have to make themselves happy. Even if you agree to be the saviour of their relationship, that just means you are trading your happiness for theirs. There won't be any overall improvement, just a trade off of happiness. In other words, they will be happier and you will be less happy. That's wrong on so many levels.

It's better if each one of them finds an outlet, like joining a club, each one joining their own club. If they spend time apart, perhaps they will enjoy their time together more. They also need to get a hobby or some form of past time, like joining a bridge club.

I think you know you can't save them, even if you agreed to. So get on with your life, be nice to them, set reasonable boundaries, ditch the guilt and build your own life. And forgive them for being needy. And forgive yourself if you think you've done something wrong (which I don't think you have). Good luck.