The Collaborative International Dictionary of English v.0.48:

Duograph \Du"o*graph\, n. [L. duo two + -graph.]
   (Photo-engraving)
   A picture printed from two half-tone plates made with the
   screen set at different angles, and usually printed in two
   shades of the same color or in black and one tint.
   [Webster 1913 Suppl.]