Exactly what Penny said.

Writing is collaborative so the more you work together and the more you know about each other's characters and intentions the better.

If you're doing a 1x1 with me and could write a post as my character, and I could write a post as your character, and the change would be neigh  indecipherable we're probably going to be amazing partners.

Whether it's a one on one fight and we're deciding if it's going to be a Respectful Standstill that leads to Mutual Admiration or a You Just Barely Got Me With, I'll Be Watching Next Time that leads to a Friendly Rivalry or a Full Tilt Drag You Down and Stomp You Out grudge match that leads one to seek revenge on the other.

Or if it's a Romance but X has an unrequited love for Y. Or X wants Y and Y wants X but X can't leave W because Ws mom just woke from a 14 year coma and she's pretty sure Z, the pool boy, put Ws mom into the coma because Z is secretly Ws mom's illegitimate son from her brief dalliance with T the Brazilian Yoga instructor who was oh so flexible.  Or X and Y are into each other but both are nervous/shy.  

Ex:
I had a 1x1 planned with TyrannosaurusRex and we had talked out our characters backstory and doubts and fears and like character flaws.  It helps a lot.
If you partner knows roughly what your character will want out of a scene, and you can clarify exactly what your character wants and how attached you are to that character getting it, you can really set each other up.  Working at a shared goal rather than at cross purposes.