Having two parts.
Having two participants; joint.
Having vertices that can be divided into two independent sets (see bigraph)
Divided into two at the base.
doubled
valvate with the margins curved outwardly
folded, with the abaxial surfaces facing one another
To double again: to multiply: to repeat.
To repeat (a word or part of a word) in order to form a new word or phrase, possibly with modification of one of the repetitions.