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It's not known, or even knowable.
In English, its immediate ancestor was Old English "twá" / "tú" - but there are similar words all across the Indo-European language family (for example, Sanskrit "dwau" / "dwē", Greek "δύο", Latin "duo", Old Irish "dá", Lithuanian "du", "dvi", etc.)
That suggests that "two" is a word that goes right back to Proto-Indo-European, the postulated common ancestor of most of the European languages and a lot of others right across to India. And we can never know why the Proto-Indo-Europeans, who probably lived around 4000BC, coined the word.
dabbler in linguistics
Oxford English Dictionary