It's good that you can do either - so I'll show you in .ascx and you can go from there:
In the top section of the page, register SolPartMenu twice - something like this:
<%@ Register TagPrefix="dnn" TagName="SOLPARTMENU1" Src="~/Admin/Skins/SolPartMenu.ascx" %>
<%@ Register TagPrefix="dnn" TagName="SOLPARTMENU2" Src="~/Admin/Skins/SolPartMenu.ascx" %>
Then put your menus in place using those TagNames you registered as the "id=":
<dnn:SOLPARTMENU runat="server" id="dnnSOLPARTMENU1" Display="Horizontal" SeparateCss="True" rootmenuitembreadcrumbcssclass="MainMenu_MenuItemSel" rootmenuitemactivecssclass="MainMenu_MenuItemSel" rootmenuitemselectedcssclass="MainMenu_MenuItemSel" submenuitemselectedcssclass="MainMenu_SubMenuSel" submenuitemactivecssclass="MainMenu_SubMenuAct" rootmenuitemrighthtml=" " />
<dnn:SOLPARTMENU runat="server" id="dnnSOLPARTMENU2" Display="Vertical" SeparateCss="True" blah, blah...
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To do this using HTML/XML, you'd need to have two separate SolPartMenu <object>'s listing each needed parameter (Display, SeparateCss, etc.) in the xml file, but in this case you would use SOLPARTMENU:1 and SOLPARTMENU:2 so the parse engine knows what to do.
Hope this helps!