Mount drive with two drive letters instead of one

It is not possible. The limit is refered to in this Microsoft support page:

http://support.microsoft.com/kb/307889

Because mounted drives are not subject to the 26-drive-letter limit for local drives and mapped network connections, use mounted drives when you want to gain access to more than 26 drives on your computer.

It is not supported as such. However, you can use subst command to create new drive letter out of folder. If you do subst on root folder, that would give you desired results.

E.g.

SUBST X: D:\