twain vs span

twain and span both are nouns.

Word	Noun	Adjective	Verb	Adverb
twain	Yes	No	No	No
span	Yes	No	Yes	No

As nouns, twain and span are synonyms defined as:

Other synonyms of twain include brace, couple, couplet, distich, duad, duet, duo, dyad, pair, twosome, yoke.

twain (noun)	span (noun)
two items of the same kind	the act of sitting or standing astride
	a structure that allows people or vehicles to cross an obstacle such as a river or canal or railway etc.
	the distance or interval between two points
	a unit of length based on the width of the expanded human hand (usually taken as 9 inches)
	two items of the same kind
	the complete duration of something

twain (verb)	span (verb)
	to cover or extend over an area or time period