This can be solved with standard IDEs. Putting two modules or classes into a single file pretty much guarantees a level of coupling. This does not reduce complexity.
I've used IDE's that make this more or less painful, but none that actually solved it. If anyone has any suggestions on one that does, I'd be interested to try it out. I don't really care what language. I can pick up enough to see what it feels like.
I also want to say that rich hickey talked about a file as a unit of code not being very good, but I don't recall where, or if he really said it. I want to say it was in a Datomic podcast right around when details about it were coming out.
http://thinkrelevance.com/blog/2012/10/12/rich-hickey-podcas...