Cleave: if you deal a creature enough damage to make it drop, typically by dropping it to bellow 0 hit points or killing it you get an immediate, extra melee attack against another creature within reach. You cannot take a 5 foot step before making this extra attack. The extra attack is with the same weapon and the same bonus as the attack that dropped the previous creature; you can use this ability once per round
Two Weapon Strike: When armed with slashing weapons in both hands, you can make a special attack once per round using both weapons. You combine your best primary hand attack with your best off hand attack, making a single attack roll based on your best primary hand attack bonus (as modified as usual for two-weapon fighting). This attack roll determines the result for both attacks. Determine threats and critical hits individually for each weapon, but otherwise as usual. If both attacks hit, total up the damage done and apply it as thought it were a single attack for all purposes, including punching through damage reduction and causing death by massive damage.
“The extra attack is with the same weapon and the same bonus as the attack that dropped the previous creature”
Here is where the question comes in, if you dropped the opponent with 2WS (Two Weapon Strike) you used both weapons to drop the opponent not the primary not the secondary, both,
I would probably rule as Voltumna mentioned but it would be more because as a DM I would think this to be to overpowering (and I tend to be a hardass when I DM :roll: ) on the other hand this is essentially the same as the character using a 2 handed weapon an in that sense so comparing it would not be overpowering except that the guy with 2 weapons needs one more feat to do the same