The joint looks fine to me. You just need to work on accuracy when setting up your router. If you don't have a router table you might consider doing it on a table saw instead. It helps if you work from one side of the boards instead of making one cut and turning it around and run the other side. To do that the wood would have to be precisely the same thickness which is hard to do with wood. If you mark both your pieces one side or the other the face side and run only the face side against the fence I think you would find the joint a lot easier.