It’s a pretty cool process. If each of you bring characters to a table and sits them down for a chat, you find that the characters' reactions become real reactions. They are true conversations between two separate minds and they are more dynamic – not to mention more fun to write!
Another writer also acts as a check-and-balance against your ideas. You will think harder, and test your thoughts more, before you write them down. It results in a story that is told better and more believably.
What about throwing off your 'writer vibes'? If you’re writing something very nebulous and poetic, perhaps team-writing is not the best idea. But if it's a story of people interacting with people, give it a shot – it’s worth it, if you can attain a working partnership with another author.
All that to say – here is a ‘test’ I co-wrote with another author (I’m waiting on her pen-name, standby). It’s very simple; just a ‘what if our characters met’ scenario. It’s short and begins and ends very abruptly – but can you find the seams between my writing and hers?