Dual-wielding plus shields

[steve hick]

AFAIK Marozzo is the first to cover two swords of equal length. This apparently was popular as a method in the middle 1/3 or first 1/2 of the 16th century. It's an allowed pairing IAW French judicial duels. There are several examples of cased rapiers (sic) from this time.

As had been said elsewhere not a style you see in earlier manuals.

Steve

[Casper Bradak]

I definitely second that. It rules out most grappling, and a huge amount of the techniques in the manuals. It would be difficult to use mounted to say the least, and you would certainly be at a disadvantage trying to fight an armoured opponent in that fashion.
As an off-hand defensive weapon it is expensive, outclassed by a more versatile (and durable) shield, buckler, or even a dagger. It would get in the way of most techniques, and if someone grappled you while you were trying to use two swords, you'd more than likely have to drop at least one anyway. The reach is shorter than a longsword, the techniques more limited than a single sword, it's just not generally practical.