Everything with HMVC (and modularization in general) tends to come down to what Narf said. If your module works on its own, you're fine. If your module requires another module in order to work properly, then something's wrong (I'm guilty of this myself in some of my older code).
If you do have a dependency on another module, but can't merge the dependent modules for some reason, they need to work properly without the other module(s). What this means depends on what the modules are doing for one another, but, in the end, it comes down to either failing gracefully or providing another means of fulfilling the request.
