Both Section 508 and the WCAG state that, as a last resort, a separate, accessible website must be provided. As a last resort. Meaning a second site is created if you absolutely cannot achieve accessibility in any other manner.
Wow. If you have to create a mirror site, it makes me ask just what, exactly, is going on with your site in the first place? The sheer amount of effort required to make a site so inaccessible that a second site is required boggles my mind. I can only imagine the effort and maintenance going into this endeavor.
Yet, many developers and their clients immediately assume a second site is the first and best option. After what I can only assume is a quick perusal of the guidelines, it is determined that two sites are better than one.