One of the biggest issues when it comes to the writing, is that two people will very often have two different writing styles. This means that either a lot of editing will be required to set the same tone across all chapters (ensuring verbiage, sentence structure, etc is consistent), or that both people come up with the ideas of the story but only one person does the actual writing of it.
in my experience, it is quicker if only one person handles the actual writing.
That said, it can be very difficult sometimes when you clash on how a particular scene should unfold, the way in which a certain mechanic should work, or the direction a specific character should evolve.
I would recommend having a very frank conversation with the person you consider partnering with ahead of time, and hammering out some details about the how you'll proceed, before you talk about the what that you'll write. Ensure you're both on same page with expectations of the experience.
I've read a few published works by a pair of authors, and occasionally I get one where it is obvious who wrote which sections. As a reader, I'm pulled out of the story when the author's style suddenly changes. That's just me, though.
I can't imagine co-writing something. I've got a pretty clear picture of what I want and I'm not sure adding another opinion would add to the story. That doesn't mean I'm not open to feedback, but an equal voice is a bit too much.
That being said, it can absolutely work, though usually if there are two distinct parts to a book, like two main characters alternating chapters.
As for world building, I love it. Would I create something just for someone else to use it? Sure. For free? Hell no! For a review? Still no.
But if you can find a friend to do it, go ahead. It will also give you someone to bounce story ideas with