Definition 1 of 1
A 'two' happens every time a kid loses a game or gets a lesser share of the goodies or gets teased hurtfully.
It is a permanent and lifelong status declaration at that moment, which can't be reversed even by god, except until the child declares a 'Say!' status, which could be a moment later depending on the benefits of a status change.
Probably it means we are not 'one' anymore so, 'two'.
The declaration is accompanied by gesticulation with the tips of index and middle fingers locked into an elongated loop through which the child blows air in the direction of the newly declared enemy.