Today a friend of mine and I sat down (for the second time, actually) and wrote together. We're both working on NaNoWriMo, and we're both doing, well...Something! He's further behind than I am and that's all I'll say because he's the sort of guy that can bang out a bajillion words in one burst when it suits him. Not that I'm not, or at least not that I didn't used to be. I've actually been fairly steady with this NaNoWriMo project! How steady?
Obligatory Word Count: 28,085.
On topic, working with a friend can be a blessing or a curse. When you write with a friend, and you're each working on your own projects, you can bounce ideas off of each other. That's a huge blessing - if you're honest with each other you can shoot down bad ideas while building up the best of them. You can build concepts together - one person might suggest the first half of a solid plot arc, and the resolution might be suggested by the second.
The curse? You have to stay focused. "Writing with a friend" can quickly turn into "bullshitting with a friend." There's nothing wrong with having some laughs, but if all you do is talk about work/school/women/men/etc, you aren't writing. There needs to be some serious focus involved. There needs to be encouragement and good times, but there needs to be progress.
Most of all, you need to make sure your artistic styles mesh. I often like to write to the tune of music. So does my friend. With headphones, this can work - but if I really feel a need to keep the bass thumping, its going to be problematic. It'll be distracting. If I feel like breaking my writing up into small spurts of productivity, I have to show enough restraint to avoid distracting my friend if he's writing in one long, sustained effort.
The key of writing with a friend, therefore, is to know the friend you're writing with. That's the essence of it all - that, and, of course, to enjoy your writing!