I had a look around and couldn't find anything related this, so here it goes:
Is it possible to create a widget that goes over both sidebars on top of them, a bit like thesis? And if yes, how would I go on about to create it? Thanks, SY

You could create a widget area that is over teh top of the sidebars and center area (use a new widget area at the bottom of the header), but I cannot think of a way you could have a widger 'span' the without making coding changes.