Let's see...
Dual wielders need the netbook of feats and masters of the wild.
For a double weapon fighter take double weapon fighting (nbof), power double weapon fighting, and improved power double weapon fighting for 1.5x strength modifier on both ends. Add weapon focus, specialization, improved critical and bonded weapon for +3 to hit and damage with both ends all day, all night.
Then throw in improved two weapon fighting and greater two weapon fighting for 7 attacks per round. Then get power attack and cleave so if anyone of those 7 drops someone you get an 8th. Then get boots (or a ring) of speed for a 9th attack. And finally take frenzied attack from AEG's War book for a 10th attack.
Then marvel at the whirling dervish of destruction who in addition to defending himself and maneuvering for position, managed to land 10 attacks in a round without a single level of ranger or monk. Now that's 12 feats, so this is going to have to be a fighter most likely, but that's ok since 7 of the feats are bonus fighter feats. And let's throw improved weapon focus from quintessential fighter in there for 8 bonus fighter feats out of 13 total feats. Still got feats to spare.
But at this point I want to see his attack bonus.
+5 weapons on both sides, keen if you want to splurge but probably unneccesary unless you are wielding a double scimitar from Dragon Magazine. +20 BAB +8 (?) STR +5 Enhancement +2 Improved Weapon Focus +2 Bonded Weapon -2 Light Offhand Weapon -5 Frenzied Attack = +30/+30/+30/+25/+25/+20/+20/+15 with an extra partial action of +32 (no TWF penalty because no full round action to fight with both ends), +30 if any of those first 9 hits drop someone.
Damage would probably be 1d8 +12 STR x1.5 +5 Enhancement +2 Specialization +1 Bonded weapon = 1d8+20 on both ends.
I guess if you wanted to get really silly, and with 760k at 20th level, silly is guaranteed, you could enchant both ends with speed for 12 attacks an average round for an average damage of 294.
Pretty weak, but then 26 strength for a 20th level melee fighter is really conservative.