Building a relationship takes time as well as tons of effort. Being intentional about building a relationship is important. Mindfully going into co-teaching will benefit both teachers and the students. Some people are not naturally comfortable with having another person in their classroom, so easing into co-teaching may be necessary.
To begin with, meet ahead of the year and get to know as much as possible about one another. In my experience, there have been teachers who are more open to sharing and others who are less. That's okay! As long as your co-teacher knows you care and you are interested, that's a start.