A variant would be [monocephalous][1]:

>    1. Having only one head; in botany, bearing a single capitulum or head.
>    2. Specifically, having the character of a monocephalus.

or [monocephalus][2]:

>    – noun  
>
>    1. In teratol., a double monster having only one head but two bodies.
> Also called syncephalus.

See also [syncephalus][3]:

>    – noun  
>
>    1. In teratology, a double monster with more or less fusion of the heads:
> same as monocephalus.

  [1]: http://www.wordnik.com/words/monocephalous
  [2]: http://www.wordnik.com/words/monocephalus
  [3]: http://www.wordnik.com/words/syncephalus