This is what I mean:
The idea is that one navigation is the main one and the other one a second level.
Is it a bad idea? If so, why?
Also, which navigation should be the main one? I'm inclined to make the top nav the main one and the sidebar the second level, so that if it's in fixed position (see the previous screen scrolled down here) the user can scroll down and still move through the same application module without having to scroll all the way up.
The only downside I view in using the top nav is that it doesn't scale a lot (especially for back-office applications which could have lots and lots of modules), but I think we can handle it.