Common wisdom dictates 'two heads are better than one'. Working with someone has many pros: you can do less of the work, having someone else invest in your work takes away a lot of loneliness, and with a co-author, there's a chance you won't both get writer's block at the same time.
On the other hand, you can't control timing of the manuscript perfectly, and you're going to have to worry about someone else's conflicting ideas competing with your own. Many co-authorships happen based on existing friendships and mutually-formed ideas, but maybe you're finding yourself publicly searching for a someone as a co-author for a book you have in mind. If so, here are some things to think about.
1.Common thematic goals
Most of the time, our worldviews come out at least a tiny bit in how we write our characterizations and themes. If you and your co-author have wildly different worldviews, you have to plan for that at the outset. Is this book going to reflect the struggles between two main characters with opposite worldviews, trying to make peace with each other in a polarized world? If so, your opposing co-author makes perfect point of view material, if you can pull it off without booking a flight to Paraguay in a rage (Paraguay is the only country in the world where you can legally duel someone).
2.Unified writing Style
Are you going to break your book into chunks representing different POV? If not, how will you make sure you both write the same characters with the same voice the whole time? This is perhaps the trickiest aspect of working with a co-author. If you don't break into different POV chunks by author, or somehow find some other way to apportion the work for a consistent voice, you will have to spend a long time talking about the book and characters before you both get on the same page. On the other hand, you might enjoy that; getting all worked up about planning might be almost as fun as the actual writing.
3.Putting in the work
Will your co-author put in the same amount of work as you do? Would you rather he didn't? If you don't break the book into POV chunks, who will do style checks? Don't choose a co-author just because you feel he or she should get in on the project, especially if you already have your ideas all laid out. Choose a co-author who can put in an amount of time you're comfortable with--not too much, so that she dominates the project, and not too little, so that you're left to do everything.
If you're meant for co-authorship, you and your copilot will throw ideas together all the time, really enjoy them, and just make it happen. Sometimes these things are natural; sometimes you can structure them. Don't stress the process more than you need to.
Some AuthorHouse authors like John P. Lopez teamed up with former NFL star Dan Pastorini to write Taking Flak, a memoir about the legendary sportsman's life. The latter's life story couldn't have been as richly told, otherwise.