I have written and sold one collaborative novel, and I'm in the middle of writing another, so I have some experience in this sub-specialty of our craft.
Once you've gotten past the "Let's write a novel together! It'll be fun/great/a ball!" stage, reality sets in. First of all, forget the idea that it will be less work. It will take more time and energy total between the two of you to write something than it would if one of you wrote it solo. You'll be fortunate if it only takes 150% as much time and energy as a solo work. Second, this will be different from writing a solo work. Trust me. Here are some of the practical matters you will need to deal with. Some of the points are my own observations, and some are gleaned from other authors who do frequent collaborations.
1. Check your egos at the door. Really. You are establishing a relationship here, and although you may or may not be equals in talent, knowledge, skill, and drive, you need to be on a personal basis of honesty, diligence, and compassion. The old teaching of "Treat others the way you want them to treat you" comes into play.
2. Determine your collaboration approach. To steal from my May 28, 2012 Fictorians article "Anatomy of a Collaboration," you need to settle on an approach like one of these:
This step is where you agree on how the byline will be styled. If it's a senior/junior relationship, the senior author's name almost always goes first. This is also where you agree on how the revenue (and any expenses) will be shared. And even if you're friends, write it down. It will save grief later, I promise.
3. Decide who the tie-breaker will be. If you arrive at a point where the two of you are in disagreement about something serious and you can't continue until it is resolved, someone has to break the tie. Determine who that person is at the beginning of the project. It may be a senior author. Or, if you're writing in a universe created by one of you alone, then that person will probably be the tie-breaker. But regardless of who it is and how you determined who it will be, if it ever has to be invoked, remember Rule 1 - check your egos at the door.
4. Do any world-building that has to be done that will be foundational to the story.
5. If both of you are outliners, you'll need to write an outline. If one of you is a pantser, you'll need to write an outline. If both of you are pantsers, you'll really need to write an outline. Seriously. If for no other reason than to keep you both facing the same direction. Especially if you're doing the "you write this part and I'll write that part at the same time" thing.
6. Communicate, communicate, communicate. Especially about the important stuff, but since it may be difficult to know what will be important twenty chapters down the line, it's mostly going to be important stuff.
7. Again, communicate, communicate, communicate. If you're the junior author or you're working in someone else's universe, don't be afraid to ask questions. And if you're the senior author and/or the universe creator, don't brush your partner off.
8. Remember Rule 1.
9. For the third time, communicate, communicate, communicate. If there's one area where collaborations can really be more difficult than solo work, it's flexibility in dealing with change. When you're working on your own, if you get a brilliant idea when you're 80% done with the work, backing up and rewriting twenty chapters is not so much of a much. When you're collaborating, however, especially if you're using one of the parallel streams-of-creation methods, your idea may blow up your partner's work in a big way. So before you do anything with your Grand New Idea, talk about it-in-depth and in detail. If the decision is Do It, you revise the outline. You write it down so you can both be in agreement as to what the change is, what the effect is, and who's doing what to implement it. If the decision is No, you continue down the existing path with no looking back.
10. Remember Rule 1.
11. Set deadlines as to when milestones will be accomplished. You may or may not attain them, but if you don't set them, this thing could drag on for a seeming eternity. As much as possible, hold each other accountable.
12. Remember Rule 1.
13. When the first draft is done, review it together. Decide what needs fixing, and determine who will do it. Execute the fixes.
14. Determine early on who will do the final polish to smooth out the edges and establish a consistent voice. This will usually be the senior author, the writer who owns the universe, the person who's the better editor, or whoever won/lost the coin flip.
15. And finally, remember Rule 1.
Okay, that's probably not everything that needs to be thought about, but it covers the high points. Good luck!