I see there're two versions of two-weapon rend that coexists in DND 3R:
Two-Weapon Rend [General]
You wield two weapons with an artisan’s precision. Each strike builds on the next, allowing you to deal more damage.
Dex 15. Two-Weapon Fighting, base attack bonus +11.
If you successfully hit an opponent with both of the weapons you wield, you deal extra damage equal to 1d6 + 1-1/2 times your Strength bonus. This extra damage is treated as the same type that your off- hand weapon deals normally for the purpose of overcoming damage reduction and other effects related to damage type. You can gain this extra damage once per round against a given opponent.
A fighter can select Two-Weapon Rend as one of his fighter bonus feats. A ranger who has chosen the two-weapon combat style can select Two-Weapon Rend as long as In-has a base-attack bonus of +11 and is wearing light armor or no armor.
Two-Weapon Rend [Epic]
Dex 15, base attack bonus +9, Improved Two-Weapon Fighting, Two-Weapon Fighting.
If you hit an opponent with a weapon in each hand in the same round, you may automatically rend the opponent. This deals additional damage equal to the base damage of the smaller weapon plus 1 ½ times your Strength modifier. Base weapon damage includes an enhancement bonus on damage, if any. You can only rend once per round, regardless of how many successful attacks you make.
If I pick both, does it mean I can rend twice a turn? Or would one of them just become useless?