I agree with Joe, this is something that should be done with extreme caution, if SEO is of any importance.
You could avoid using multiple-sites altogether if the content is truly the same on every site. Instead, use the URL Rewrite module for IIS to rewrite the domain to include the skin=NameOfSkin query parameter. For instance:
MyCoolUrl.com needs to use a skin named my-cool-url so you would use URL Rewrite to change MyCoolUrl.com to MyCoolUrl.com?skin=my-cool-url. Users would only see MyCoolUrl.com.
I've not tested this but in theory it should work okay.
Hope this helps,