Every writer at some point entertains the idea of working with a partner. And the beginning-er the writer, the worser the reasons. Ultimately, there’s only one good reason to partner with someone, but let’s start with five bad ones.  Spoiler: they all boil down to some variation of you think it will be easier to write with a partner. Hahaha. No.
#5—The Idea Guy
You’ve got a great idea for a novel, a screenplay, a whatever. All you need is a writer monkey to type it up for you. You can split the profits.
This is such a cliche that it’s amazing it still happens. And yet it does. The Idea Guy is bringing nothing to the table in this scenario. Zero. Zip. Nada. Writers aren’t short of ideas. They’ve already got more ideas than they can write in their lifetime. Why would they give up half the money for an idea they’re not even particularly invested in?  This is maximum arrogance and laziness. 
#4—The Dick Van Dyke Show
You and your funny friend are always cracking each other up. You just need to write that shit down. 
Hmm. Are you also swapping well-rounded characters and compelling plots? Because even a laugh-a-minute sitcom needs those. Oh, and hey, do you ever share those jokes with other people? Are they funny outside of the context of your years of friendship? Because it’s a lot harder to write for an audience than for an individual. 
#3—The Division of Labor
If you each write half the novel, you’ll be done in half the time.
Yeah, no, it doesn’t work that way. Novels are like babies—you can’t divide them up for efficiency reasons. The Bible tells us that. Unless you’re planning the literary equivalent of Exquisite Corpse or a story-collection set in a shared world, each writer needs to be up to speed—and in agreement—on the whole story.  Adding a writer means you’re actually creating more work by adding communication and overhead to the project. 
#2— The D&D Party
You’re a great writer, you just suck at dialog. Or description. Or story. You just need to find the the Warrior to your Mage.
Let me begin by rejecting the premise—you’re not a great writer. Not if you suck at dialog or description or story. Sure, you’ve got your strengths and weaknesses. We all do. But unless you’re already good enough to pull off a project on your own, the only synergies you experience are likely to be negative. This really comes down to a division of labor but by type of writing. Add to the list of things a novel is not—a dungeon or a dragon.
You acknowledge there’s no way to save on effort, you just want some company along the way.
Maybe? I mean, at least you’re being honest with yourself. But you’re going to go through all the effort for half the money and half the credit. Plus you’re subjecting yourself to the whims of your writing partner’s schedule, and they to yours—assuming you’re both equally committed, and neither of you is just an Idea Guy in disguise).
Okay, that’s five bad reasons to collaborate (emphasis on reasons—you can succeed with any of these modes of collaboration, but you can also sail across the Atlantic in a tub—that doesn’t make it a good idea). The single good, and hopefully obvious reason, to collaborate is to create something better than you could on your own. Bringing back the baby metaphor, you mix your creative genes, and some months later, out pops something that’s the best of both of you.
Wait—isn’t that just Bad Reason #2 with a positive outlook? Not if each of you is good enough to succeed without the other. The motivation becomes excellence, not fear.
A writing partnership, when it works, can be exhilarating. I’ve been fortunate to be part of some wonderful collaborations–but also many, many spectacular failures. Just make sure you’re in it for the right reason.
 I’m discounting the ever-present “someone’s paying you enough to make it worth your while.” By definition that’s true of everything.
 Unless you are five-years old and your 26-year old brother is a professional comic book illustrator. And even in the case of Axe Cop, I believe the partnership suggestion went the other way around.
 I know how annoying this is because I’ve been on the receiving end of it. So it’s painful to admit that I have also been the Idea Guy (though, to my minor credit, not for a split of the profits). It’s much, much easier to tell another writer what they should be working on than it is to sit down and write it yourself.
 The Dick Van Dyke Show was about a couple of friends who were comedy writers and basically pushed this narrative. One of the greatest sitcoms of all time (don’t take my word for it), but it aired in the early 60s, so I don’t blame you if you’ve never heard of it. I really need some recent references. (MST3K? Is that still a thing?) But check these credits: created by Carl Reiner, starring Dick Van Dyke (obviously) and Mary Tyler Moore. The writing staff included Joss Whedon’s grandfather.
 I actually tried this with an improviser friend. It was fun! We had a blast cracking each other up. But most of the jokes were about how poorly the actual writing was going (funny because they were true). We never finished anything.
 Yep, tried this too. Here’s the first awkward question—how the hell are you going to divide this baby up? By draft? By act? By character? In my case, we alternated chapters. Or tried to, because, obviously, it didn’t work.
 The Mythical Man-Month by Fred Brooks, a post-mortem of IBM’s OS/360 development process and a classic of software engineering literature, demonstrates convincingly how adding resources to a project can actually slow it down.