Words for two or twoness

• Two derives from an old Germanic word, which itself comes from an older root which supplied many of the ancient and modern European forms, such as the French deux, the German zwei, the Dutch twee, the Latin duo and the Greek duo.
• Dual, double and duplex plainly come from the Latin duo, and the prefixes bi- and di- derive from a similar root5.
• Twin derives from the Germanic root, as do words like between and even twig, due to the sense of it being a forked stick.
• Pair can be traced back to the Latin paria (equals), as it implies two items which are equally matched.
• Brace in terms of, say, a brace of pheasants is less clear. It certainly comes from the old French word brace meaning 'arms', but the word then goes on a bit of a journey. It could be that the word 'embrace' gave us the verb 'to brace' implying to tighten something, then this was applied to things that tighten, like straps. It is also known that a pair of hunting dogs on leashes was known as a brace, and this may have then been applied to two animals of other kinds.
• Couple comes via the Latin copula (a tie or connection), and was first applied to a married couple before it became a general synonym for two of something.
• A tandem, the bicycle made for two, is itself a Latin word meaning 'at length', suggesting in a sense one behind the other. It was originally applied to a carriage pulled by two horses in that configuration.
• The English ordinal version of two, second, is unusual in that it's not connected with the word 'two' as it is in other languages, like the French deuxieme and the German zweite. There was no Old English word for 'twoth' at all; instead they used 'other', but as this was ambiguous adopted the word 'second' instead. This word came via French from the Latin secundus, meaning 'following'.

See http://www.bbc.co.uk/dna/h2g2/A45901749