Kate and William? If ever I talk about them with anyone, they're Kate and William. I was out last week with three friends and one of them brought them up in conversation "The Duke and Duchess of Cambridge ...." If she's feeling relaxed she might refer to her as Catherine.
Yes I know it is their official title(s) but it's a job title, not a name. And it feels weird to talk about someone in that way. I wouldn't talk about my boss as "the head of department", I would say Mark. Because that is his name.
I'm roughly the same age as Kate and William too, and I sort of "grew up" with him in the news etc, then when Kate came on the scene, she was just an ordinary person. I can't bow and scrape to either of them, in person or in conversation. That isn't disrespectful. Indeed, at the end of the day, they are simply people just like us (this is not a post to debate the usefulness of the royal family, if they're scroungers or lazy or should be cancelled, that's a whole other debate). William does call her Kate in private and has been overheard saying it at events.
So, Kate and William it will continue to be if I ever discuss them.