If they are co-dependent, then that, by definition, means that they are not independent. What value do you get by having co-dependent modules that you can't get by having a single module? If you want two public APIs with one common implementation that is co-dependent, you can have three modules where two depend on the co-dependent part. Without seeing a motivating use-case, I'm just going to assume that F# is succeeding in preventing you from making a bad design decision.
