Here is how to add multiple document templates to a library.
Select Yes for Allow Management of Content Types under:
Library Settings -> Advanced
Then Go to Site Settings -> Site Content Types -> Create
Type a Name and description.
Under "Select parent content type from" select -> "Document Content Types"
Under Parent Content Types select "Documents"
i Prefer to use a Existing group called "Custom Content Types" under groups.
Then you can set the Document template.
Go to Advanced under the Site Content Type page which opens after you clicked OK in the last step.
The you can upload your new template on this screen.
Then go to Library Settings -> Content Types and select "Add from Existing Site Content Types"
Select the Content type you just created and click Ok
And that is it. You will now have an additional Template under the new document Drop Down.