mojoPortal comes with a lot of skins with different menu styles. One of these skins (andreasviklund-02) has two sets of horizontal menus allowing for a more compact menu system on 2nd tier pages. You can check out the above mentioned skin using the demo site for a working example. The mojoPortal site itself uses a similar menu system so you can use it as an example also.
I am currently working on my church's website (http://acckc.org/mojo) and it uses this menu style.
To achieve this style of menu, you must add a few controls to your layout.master file.
Place this code under the HTML <form> tag with the other SiteMapDataSource controls.
<asp:SiteMapDataSource ID="level3datasource" runat="server" ShowStartingNode="true" />
Change the PageMenu control to the following and then move it to the desired location within the layout.master file. This control will show 2nd tier pages.
<portal:PageMenu id="PageMenu1" runat="server" TopLevelOnly="true"
UseTreeView="false" HideMenuOnSiteMap="false" UseSpanInLinks="true"
Direction="Horizontal" DynamicDisplayLevels="0" />
Finally, add a second PageMenu control. This control will show 3rd tier and lower pages.
<portal:PageMenu id="PageMenu2" runat="server"
StartingNodeOffset="2" SiteMapDataSource="level3datasource" />
For skinning of the menu, please refer to the stylemenu.css file in the i7MEDIA-2horizontalmenus skin.