From compedium, rapier is finesable so it is light wepon and OTWF does nothing for it...Oversized Two Weapon Fighting
Usage: Passive
When wielding a one handed weapon in your off-hand, you take penalties for fighting with two weapons as if you were wielding a light weapon in your off-hand.
On the other side if you would go 2x eFalcata ICS and WP: pesh you would have use of OTWF...