Finding a creative partner, whether it be for writing (or anything else) is easy. The challenge is finding someone who elevates your work. Like the classic Tennis suggestion: Always play with someone better than you.
If you’re a writer, it’s best to know what your strengths and weakness are before finding a writing partner, then you’ll ideally partner with someone who is strong in areas that you’re not. If you are good with plotting and characterization but weak at titles, pacing and dialogue then you know what your partner must excell at etc etc.
Ideally, like any other relationship, it’s best to try collaboration with several different people. Perhaps having 4 or 5 different writers that you work with on different projects will help make progress more efficient. Some writers pair with an “expert” in the arena they’re writing about, I’m not particularly enthusiastic about these matches because they seem to do more for the expert than they do the writer and it’d be better if they were consultants on the project and separate from the writing process. I’ve often seen writers who are particularly good at working with novelists to adapt their works. This kind of experience can be an asset when a novelist insists on having a hand in the adapting of their work.
As with any partnership, it’s hard to know in the beginning how it will work out, if at all. All writing partnerships are difficult to maintain and require a lot of investment. Often it’s more efficient to go it alone and instead use a creative group, consultant or even life coach for advice and moral support.
If you do partner up, or are exploring writing partnerships, try to limit your party to 2. A writing team of 3 or more can be a bit unwieldy. Scheduling, personalities, sharing of revenue, etc. make for just a few of the challenges. Also, see the WGA website about writer collaboration agreements. Their agreement template is an essential tool for case by case writing collaborations that may yield long term partnerships. If possible, try to structure a partnership on a 50/50 basis in terms of efforts and financial split. It helps to put you both on the same side of the table when attacking the world, and that often makes a big difference when there are bumps in the road.
About the creative team breakup. If it happens. One thing I always hear from executives, producers, agents (everyone) is: Who is the talented one? Mistaken or not, this happens. So, keep an exit strategy in mind. Always have ideas and projects you can work on solo or with an alternate partner, if possible.