double block in the same line on sidebar ?

What you need to do is to place two blocks next to each other (one above the other) and then find their unique css id.

For instance, if you want to place the "who's online" and the "who's new" blocks together, try something like this (create a new file called local.css and put this in there):

#sidebar #block-user-2 {
  float:right!important;
  clear:none!important;
  width:135px!important;
}

#sidebar #block-user-3 {
  float:left!important;
  clear:none!important;
  width:135px!important;
}

the block-user-2 and block-user-3 are the css ids for the two blocks.

(each block being 135 pixels wide)

If your putting it in the right sidebar, I found I had to use this with the latest 6.2 dev release.

#sidebar-right #block-user-2 {
  float:right!important;
  clear:none!important;
  width:135px!important;
}

#sidebar-right #block-user-3 {
  float:left!important;
  clear:none!important;
  width:135px!important;
}

yes, in the 2.x-dev branch, there are some changes to the css classes (and more)