Normal Two-weapon Rend (as per PHII) allows 1 rend attack per round per opponent. This allows you to focus those rend attacks all on a single opponent, rather than dividing them all up amongst a group of opponents.
I couldn't let it be anything less than an Epic feat.
Epic Two-weapon Rend [Epic]
You wield two weapons with an artisan's precision. Each strike builds on the next, allowing you to deal more damage.
Prerequisites: Dex 25, Greater Two-weapon Fighting, Improved Two-weapon Fighting, Perfect Two-weapon Fighting, Two-weapon Fighting, Two-weapon Rend, base attack bonus +21.
Benefit: 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 gain this extra damage once per round per successful hit with both weapons against a given opponent.
The following is Two-weapon Rend from PHII
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.
Prerequisites: Dex 17, 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 as your off-hand weapon deals normally for the purposes of overcoming damage reduction and other effects related to damage type. You can gain this damage once per round against a given opponent.
[ONCE PER ROUND AGAINST A GIVEN OPPONENT] <----This part is the primary difference.
This means that each time you hit a different opponent with both weapons, you get a free rend damage.
Now...if they had said...
[ONCE PER ROUND]
...then it wouldn't allow for multiple rends.
