The S-Sign add-on for S-Docs lets you send multiple documents in a single e-signature request. To send multiple documents, go to your Site settings and add the VF page SSMultiSign to the list of VF pages for the site: Setup > Build > Develop > Sites > [Your S-Sign Site] > Site Visualforce Pages​ > Edit and add the page SSMultiSign to your site's Visualforce Pages to enable this feature. Recipients of this e-signature request will be directed to an intermediate page that lists all of the documents to be signed.

Learn more about S-Sign here.