Split menu?

What problems are you running into with WP Nav Plus?

You could have truly independent menus. While they could have the same items as the related submenu, it’s not a requirement. You could alter the header template to conditionally load the correct secondary menu based on the current page.

Alternately, query for menu items belonging to the main menu whose parent item is a specific top level item, i.e. all of the submenu items for that page. You’d then output the query results in a loop much like you’d output post titles on an archive listing. This way if you alter a submenu, the secondary menu will automatically reflect the change. If you had truly independent menus which are supposed to have the same content, you’d need to do the same edits twice.

Since I never care much for redundant data, I personally prefer the second approach. However, this approach does require much more custom coding than the first.