span vs twain

span and twain both are nouns.

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

As nouns, span and twain are synonyms defined as:

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

span (noun)	twain (noun)
the act of sitting or standing astride	two items of the same kind
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

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