As in antlers, nostrils, lobes of a walnut, paired leaves on a stem -- anything anatomic or botanic and bilateral. Can also be used of actual twins, though the usual connotation is of being identical, so less appropriate for fraternal, even though those are much more common. Can also be used of fissured organizations (the Catholic and Orthodox churches) or nations (the Western and Eastern Roman empires, Sudan, or even possibly the Dakotas). But in general, this word is mostly used in biology. Adopted around 1790 from Greek dídymos, twin/double, with a root of duo, two.
The doe stepped into the clearing, followed by didymous fawns.