One of the hardest things to do in writing is to get two writers together, come up with a complete story, and still have it be under ten thousand words. I've started out with an outline that is supposed to be fifteen thousand end up being thirty-four thousand words. True story.
With a writing partner, you can watch each other's flowery descriptions and, in general, see how many times you can piss each other off before you call it a night. Shara Azod and I do this all the time. She has trouble with back story, I have trouble with keeping my facts straight. We've come close to wringing each other's necks on more than one occasion, but we always manage to come up with a good story. Just takes a while sometimes.
When writing with a partner, I suggest Google Docs. You can see each other in real time, the person writing can just write and not worry about typos, and you can see when one of you gets stuck. It also has a chat option so you can brainstorm while you're writing. I often tease Shara that we need to compile our chats because they are almost as interesting as the books we write.
One very important thing to consider, the absolute most critical part, is to find a partner you mesh with. One of you will always be the dominant, or lead the story, but you should have similar interests and writing styles or you'll end up killing each other before the first book is finished. When Shara and I get on a roll, we can turn out an eight to ten thousand word book in a single day. If you find a partner you enjoy writing with, you'll find you feed off each other. Each paragraph, each chapter, will come easier and easier.