Yup, the "two" referring to masculine and feminine grammatical genders. Not the other two little witnesses that got removed. It seems that PIE had two genders: animate and inanimate which map roughly to non-o-stem masculine/feminine and neuter. Later, what became the feminine developed out of neuter plurals reanalyzed as abstract nouns, e.g., bona 'goods'.
Some folks get bent out of shape when historical linguists talk about grammatical versus natural gender, charging all kinds of PC tippytoeing, but there are many languages that have grammatical gender (ultimately from Latin genus 'kind') that does not map at all to natural (biological or sexual) gender. For example, Kiswahili (and other Bantu) languages have a whole passel (more than 10) of genders, and gender concord not only between adjectives and substantives, as we're used to in IE lgs, but also between verbs and nouns. The genders are marked with prefixes (cf. mtu and bantu 'person' and 'people').
And there's that -st again in the preposition post 'after'. Aft, after, aftest?
