Why not make it split/clone into two weapons, but it just increases the feat level of Two Weapon Fighting by one level.
No skill in TWF tree gives TWF.
TWF improves to ITWF.
ITWF improves to GTWF.
GTWF improves by giving one additional offhand attack.
And this applies only while the weapon is split.
Unsplit it's a normal sword or has a meager bonus.
