if it were the girl.. it could be Tootsie.. (sounds like two..lol)

but for the boy..hhmm... I like Deuce


Dos (means two)
Segundo (means second)
Toofer

Two is the smallest and the first prime number, so maybe Prime
Japanese
HOSHI: star
HOSHIKO: star child
Vietnamese
HAI: meaning two or second

also was looking in Belgian or Dutch terms.. but some words are just too long...
Twee means two.. but doesn´t sounds appropiate for a boy IMO