Best possible dual wielder?

[opaopajr]

Mounted Halfling with Dual Wielder feat using Lances. Halfling Luck mitigates 1s, Small size opens access to more mounts, Lance is the only 1d12 by one-hand (yet needs mount condition) and is not Heavy. Add Fighter and Barbarian to taste. I recommend a flying mount, and Mounted Fighting feat, if possible. Comes online at Lvl 4.

Vuman comes online at Lvl 1, though.

Regardless you'll get plenty of mileage before Lvl 20 capstone, multiclassed or not.