Dear availlancourt

You want to say. In normal view the submenu is closed. Once selected (clicked) the rootmenu is open and you can see the submenu. Do you mean if you click again it close?
That you can't do with the solpartmenu. But you can set in the solpartmenu a flag to force the downlevel menu to display. The values are true|false.
Default setting = false.

How can you do that?
If you upload your skin (.html + .xml)

<Objects>
  <Object>
    <Token>[SOLPARTMENU]</Token>
      <Settings>
        <setting>
          <Name>forcedownlevel</Name>
          <Value>true</Value>
        </setting>
      </Settings>
  </Object>
</Objects>

Or you can set it directly in you .ascx control.
<dnn:MENU runat="server" id="dnnMENU" forcedownlevel="true" />

Perhaps this can help too...
You can set menueffectsmouseoverexpand="false". Then the menu expand when you click the rootmenu.

Perhaps the navigator menu of bi4ce is a solution for you. But this menu have only a rootmenu and one submenu level.

Hope this will help.