I have just discovered that Twin Township, Preble County, Ohio, is not indexed in any of the 1840 census indices available to me.Other Preble County townships are indexed, but not Twin Township.The indices I've checked are:
The online census index at Ancestry.com.
The online census index at GenealogyLibrary.com.
The Broderbund (FamilyTreeMaker) 1840 census index (CD-#316).
The pages exist and images are online at Ancestry.com and at Genealogy.com, but you'll need to do a page-by-page search of the images to find someone in Twin Township.