It would work as follows:
Two horizontal bars, one on top of the other.
The top bar has the main menu, the bottom bar shows the submenu.
When you rollover a link in the top bar, the bottom bar will list the submenu for that section.
It would all be text based.
Please PM me if you're able to do this, and what you'd charge.
Preferrably something that is not too heavy in code.