Source : Webster's Revised Unabridged Dictionary (1913)

Biform \Bi"form\, a. [L. biformis; bis twice + forma shape: cf.
   F. biforme.]
   Having two forms, bodies, or shapes. --Croxall.

Source : WordNet®

biform
     adj : having or combining two forms; "a biform crystal"; "the
           biform body of a mermaid"