Hi Maarten,
The site hierarchy is basically a tree, the start node offset refers to the depth of the node from the root, you can split the menu up based on start node offset as seen in quite a few of the skins, andreas-vicklund-02alt1 for example has 3 levels of menu, 2 horizontal ones and then a vertical one, (as does this site).
But you can't split it up by some arbitrary number of nodes 1-6, 7-10, if those are all at the root level they have the same offset. You can create pages 1-6 as root pages and you can have sub menus beeath them as child pages.
Hope it helps,
Joe