The short answer is it can't be done. There is a thread at
http://forum.x-cart.com/showthread.php?t=23114 discussing this but so far no one has come up with a solution. The problem is, as long as 'lbl_site_title' remains empty, the Company name option value defined in the General settings/Company options section will be used as the site title instead of 'lbl_site_title'. If, however, you choose to assign a value to 'lbl_site_title', you will be able have different values for your company name and site title, but lbl_site_title will be shown on every page on your site at the beginning of the breadcrumb trail.