Here is an unusual double-front sided 1991 O-Pee-Chee Premier Dennis Eckersley/Dave Righetti error pair.  Great NoCal combo!

They both share the cards, each one with the correct front/incorrect back.  Each one with a smoother white front, and darker/coarser back.

Probably a printer error - should have been scrap, but made it out the back door?  They were hand-cut, and not exactly to standard specifications.  There a few creases, and scratches on each card.

Please let me know if I may help you find any individual or team, test issue, or proofs cards on your wantlist.  Thanks!