These are all the Tandems I could find in the database (list below); but of course as usual you'll tell me that all you're interested in is horror, *grumble*, dunno why I bother really.
The William Hughes book is definitely a milestone of that era (or at least the film was). Possibly the Gor things are trashy enough? No? How about the Saberhagen? I'm sure we've done this list before. The John W Campbell collection has the classiest cover I've ever seen!
Dead stylish, huh?
Lin Carter - Wizard of Lemuria
Lin Carter - Thongor in the City of Magicians
Lin Carter - Thongor at the End of Time
Lin Carter - Thongor of Lemuria
Lin Carter - Thongor Against the Gods
Lin Carter - Thongor Fight the Pirates of Tarakus
Edgar Rice Burroughs - Pellucidar
Edgar Rice Burroughs - Tanar of Pellucidar
Edgar Rice Burroughs - Moon Men, The
Edgar Rice Burroughs - Land of Hidden Men, The (Jungle Girl)
Edgar Rice Burroughs - Land of Terror
Edgar Rice Burroughs - Back to the Stone Age (Seven Worlds to Conquer)
Edgar Rice Burroughs - At the Earth's Core
Edgar Rice Burroughs - Eternal Savage, The (The Eternal Lover)
John Jakes - Mark of the Demons
John Jakes - Sorceress
John Jakes - Brak the Barbarian
John Norman - Nomads of Gore
John Norman - Priest-Kings of Gore
John Norman - Tribesmen of Gor
Andre Norton - Three Against the Witch World
Andre Norton - Warlock of the Witch World
Andre Norton - Witch World
Andre Norton - Web of the Witch World
Andre Norton - Year of the Unicorn
Joe Millard - For a Few Dollars More
Jane Gaskell - Atlan Saga II:The Dragon
Jane Gaskell - Atlan Saga III:The City
Rex Gordon - Paw of God
James Branch Cabell - Silver Stallion, The
James Branch Cabell - Jurgen: A Comedy of Justice
Poul Anderson - Shield
John W Campbell - Thing From Outer Space, The
Lester Del Rey - Mortals and Monsters
August Derleth - When Graveyards Yawn
Robert Bloch - House of the Hatchet, The
Hans Stefan Santesson (editor) - Gods For Tomorrow
Lawrence M Janifer (editor) - Masters' Choice 1
Lawrence M Janifer (editor) - Masters' Choice 2
Fred Saberhagen - Black Mountains, The
William Hughes - Performance
Murray Leinster - Operation Terror