Let's say this is your menu structure
Main01
Main01a
Main01b
Main01c
Main02
Main02a
Main02b
Main03
1. Create as skin with the menu startlevel set to the root and display no childeren (will show Main01, 02 & 03)
2. Create 3 other skins with the menu startlevel set to main01, main02 and main03. This menu can show the childern of the mainpage
3. Add hardcoded links to the other two pages to the skin
(this is not a very clean solution but works see the "OR" part)
To be honest I (almost) never use the SolpartMenu, but I don't think it's possible to set the menu's roottab.
I did this kid of stuff with
HouseMenu.
OR
I think this part of my previous post could be done with
Inventua Sidemenu (a vertical menu) with the the Source set to
[root page] and the "Current sub-tree only" option set to true
Sorry, I have no time to test now.
Hope this will help you.