You can do it with jQuery UI without another JavaScript library. Just add a function to the .resizable resize event to adjust the width of the other div.
$("#left_pane").resizable({
handles: 'e', // 'East' side of div draggable
resize: function() {
$("#right_pane").outerWidth( $("#container").innerWidth() - $("#left_pane").outerWidth() );
}
});
Here's the complete JSFiddle.