Ok. Correct me if this is wrong:

I've got two one-handed swords equipped. Same weapons.

I slash with my right hand sword. Takes 1 second, does 10 damage.

I slash with my left hand sword. Takes 1 second, does 10 damage

I attack/slash with both buttons/swords at the same time. Takes 2 seconds does 20 damage?

What is the point / difference, of pushing both buttons at the same time to do a dual blade attack (2 seconds 20 damage), if I can just spam my right hand sword attack (1 second 10 damage).

Both do the same damage over time?

Stylistic flair?

It is a power attack that will make several slashes with both swords if you hold both buttons down, it will get better with experience and purks.

As you level up you'll notice there are perks to increase the speed of dual-attacks. Which should allow you to deal more damage in a shorter time, if that's what you're looking for.

Dual wielding power attacks get an innate 12.5% damage bonus, they also get a perk that gives you 50% damage bonus with doing a dual wield power attack and can be made 35% faster. You can make them strong enough to one shot Alduin