1 and 2 columns composition at the same time

Hello,
 
I use XSL formatter and would like to accomplish this :
 
The flow is a half-page column, and sometimes I want it to be a full
page width.
 
For the main flow, I use a single column formatting because I do not
want to use the 2 columns. I have but an extra margin-right to simulate
a 2 column width but to compose into one column.
 
What can I do to "suppress" that margin temporarily and have the blocks
full page, and go back to half-page?
 
Thanks
 
Christian