advantages of dual wielding?

what are the advantages of duel wielding, most of the builds i keep seeing are sword and board... or weapon and board

20% more physical damage applied to both hits, 10% (iirc) more attack speed, and the benefit of two items worth of mods (highly relevant for some uniques, such as Doryani's Catalyst). Oh also 15% block chance.

Many builds also use a "stat stick" strategy. e.g. Hold a strong sword in one hand and a low damage mace with great utility stats in the other. If you use a skill that doesn't work with maces like reave, you only attack with the strong sword. Same applies the other way: strong one-handed mace for Heavy Strike, utility dagger like Ungil's Gauche on the offhand for the higher crit and block chance. In both cases all of the above listed benefits still apply.

By equipping two weapons you get 15% block chance and also 10% more attack speed and 20% more physical attack damage which translates to 32% more dps assuming two identical physical weapons.

The snag is 'identical', to avoid losing dps you need two weapons with similar dps which can be expensive, and skills that can hit with both weapons at once are few anyway. That's why DW builds these days most often use a main weapon + stat stick configuration, using skills that are compatible with your main and incompatible with off (Reave in dagger + mace combo is especially popular), that way you get all dual wield bonuses, global bonuses from off hand and you still need only one good weapon.

yet why do these sword and board have these massive DPS/attack speed? (videos)

Why not? Nobody says you can't have good dps and attack speed with board setups, you just need a better (read: more expensive) weapon to pull it off.

can you post a up to date video about a dual wielding build thats on par with the massive dps/aspd board and sword? i wanna check it out

Your google search is as good as mine, or just take any of those board build videos and imagine it with 30-40% more dps.

typical response, well atleast i found 1 DW shadow build that shows it can do over 300k dps but it uses a blade vortex spell so i dont know if it counts

Sorry, but it would be a waste of time, there isn't much chance I'll find two builds using similar items with equivalent passive trees and anything else would be worthless for comparison purposes.

No, Blade Vortex doesn't count.

im not saying similar, im saying on par. so far the shadow has 3 weapon/shield build that can do over 200k dps on its first page