should just be
<Setting>
<Name>level</Name>
<Value>child</Value>
</Setting>
That will make the 2nd menu always show just the navigation below the current navigation display. Your options for level are.
Root- shows everything
Same - Starts at the same level as the current navigation
Child - Starts 1 level / teir below the current navigation.
I hope that helps.
If you want your main horizontal to only show just the root / first level you can set
<Setting>
<Name>rootonlyl</Name>
<Value>true</Value>
</Setting>
That will make the top menu always show the root / base navigation useful if you trying to create a top "category" menu that then changes your side menu with the different pages under the category.
Let me know if that helps