Conway's Law and Interfaces
Conway's Law states that organizations will design systems that mirror their organizational as structure.
If you have one GUI team and one backend team, your code will end up split along those lines, possibly with some surprising duplication or inelegant data manipulation on either end.
One idea to improve the relationships between teams and achieve a more robust design, is to agree on interfaces. These can enforce a better working relationship, and be a natural communication point for the teams, while allowing autonomy within their domains.
I suggest that you take two opportunities to document these interfaces: once at the beginning and once at the end of a feature. That way, discoveries made along the way are captured, and future changes have the full context of the original communication design.
