Let’s talk about something you have to deal with when writing with a co-author. The word is patience.
When you write with someone else you need to understand they have their own responsibilities and schedules. They may not be able to write as often or as much as you are able to. This is where things can get tricky. You may be may be frustrated because you’ve written up a story and you’ve gotten half way through the book and your partner is only on the third chapter.
This is where you practice that thing called patience. A good way to do that is to talk with them. Talk about they chapters they’re working on and possible things they could mention or how to write it. Work with them to fill your time so you’re not just sitting there waiting for them to send you pieces. This will also help to make sure their pieces flow in the same pace as yours.