-2
It seems not easy to implement it based on the current Tab Menu's working mechanism. However, you can do it manually. For example, you can give each of the first sub-level LI an ID (only required for the first sub-level LI), and specify its CSS style. For example:
HTML:
<ul id="tabmenu">
<li>
Tab Menu
<ul>
<li id="i1"><a href="?11">Sub menu 1</a>
<ul> …
CSS:
ul#tabmenu ul li#i1 {margin-left:120px;}
ul#tabmenu ul li#i2 {margin-left:220px;}
Note you probably need to set the secondary level sub-menu to be left-aligned:
ul#tabmenu li.active ul, ul#tabmenu>li:first-child>ul
{
display:block;
text-align:left;
…
}