It would require some editing of your them template files to add a call to another sidebar. You’d also need to adjust the current sidebar’s and content area’s width in your theme stylesheet and (presumably) add CSS floats to one of more of the three columns.
Besides doing what Esmi sugested here are few more tips:
You have a style definition for a div called #wrapper in your css, try a bigger width in order to fit two sidebars, you can also shorten the width of #content in order to make room for another sidebar.