Ask HN: How to ensure co-founders work together in the same direction?

I am trying to find an efficient way for my co-founder and I to define and agree on requirements for our product and to execute our idea in the best way possible. My co-founder and I are both technical and unfortunately geographically distributed which hampers communication. Right now we have divided the application along natural lines with each of us pretty much coming up with our own requirements/architecture for our respective parts and discussing whenever necessary. I feel this approach is going to get us into trouble or lead to an inconsistent/poor user experience for our customers. How do you keep in sync with your co-founder and make sure you both move together in the same direction?