I'm looking for a word which basically means "separate an object into two parts", for example, boy and girl (gender), vegan and non-vegan, extrovert and introvert. I remembered that I have read that word somewhere, but cannot recall it now. It likes "bipolar" or "bilingual".
You're thinking of bisect
- to divide into two usually equal parts:
The new road would bisect a designated historic district.
- geometry to divide something into two congruent (= exactly equal) parts
However, I'm not confident that it fits the examples you gave. Bisect is used with physical things, or, in particular, geometry.
To separate into two parts is to take something single and split it in half. It's normally used in an empirical sense—splitting something physical into two smaller pieces.
But when you are talking about bipolar and bilingual, you're using it in a figurative sense. If I spoke two languages, I would still be the same person—not two people. Perhaps you could consider me as have two parts, but it wouldn't be the same thing as pouring half a glass of water into another glass and ending up with two half-glasses.
It's probably more accurate to say that if if I exhibit two different things at once (like a coin has two sides), what I represent is duality or dualism.
From Merriam-Webster's definition of dualism:
2 : the quality or state of being dual or of having a dual nature
1 of grammatical number : denoting reference to two
// a dual pronoun
2 a : consisting of two parts or elements or having two like parts : DOUBLE
// families with dual incomes
// held dual citizenship in France and the U.S.
// a dual function
// a dual exhaust system
b : having a double character or nature
1 : the dual (see DUAL entry 1 sense 1) number of a language
2 : a linguistic form in the dual
This can also be used in the sense of the juxtaposition of opposites that aren't necessarily part of the same actual thing or person but of an idea:
// the dualism of human nature
// the dualism of good and evil
Alternately, if you're looking for a word that looks the same as bipolar and bilingual, you might be thinking of bifurcate:
: to cause to divide into two branches or parts
// bifurcate a beam of light
// The stream bifurcates into two narrow channels.
However, again, this is not normally used in a figurative sense, and wouldn't be used to describe the dual nature of the ideas of boy/girl, vegan/non-vegan, or extrovert/introvert.