and one more thing, think what is practical. rapiers are mostly designed as stabbing weapons, not slashing, for the most part, I mean yes you would slash every so often, but the edges are weak. rapiers are meant to be fast weapons and in order for that to happen, they need to be light which causes the durability to not be that high.
So imagine using two arms to thrust one after another, it'll feel weird and probably not be able to get the most strength in every stab. my point is about the stab and slashes is that you cant just swing it around and kill stuff, you actually need to need to make sure the point of the blade hits the target, not the side. not to mention rapiers are weapons of reach, your trying to extend the blade as far as you can which is why people hold rapiers on their side, not in front.
For katanas, most katanas are sharp only on one side, I almost never see double-edged katanas, you have to hit your opponent on the same side and you cant swing one way then quickly reverse, if you do attempt that, you literally have to roll the katana in your hand to make the sharp side facing them.
Since the 1 handed longswords are made to be double-edged for the most part, they can easily attack from whatever part of the blade.
summary: rapiers are ranged stabbing weapons. if you dual wield them, you can only extend the range of... none of them, because you're having to face your opponent from your chest, not your arm. katanas just will be difficult to dual wield because of the skill you need to flick and position yourself in a good position all the time because it leaves you vulnerable, but it can be done. longswords are naturally easier to wield dual for the reason that the blade is effective at slashing with any part of the blade.
I didn't include gr8sword because... well... they're massive, you'd have to be FREAKISHLY strong in order to dual wield them. but if you could, dang you'd be a killing machine if you had good armor (you will always have weak spots for the reason it'll be hard to recover after swinging from sheer weight and momentum (thanks to the law of objects in motion will stay in motion unless affected by 3rd party source)).
Dual Wielding is actually more effective then single wielding a 1 handed longsword thanks to both swords attacking theres probably a higher crit rate as well but they should make so dual wielding should do like 25% more dmg then a single long 1 handed sword
.-. it makes absolutely no sense when 1 sword deals, lets say, 106 dmg thats a 1H sword and..like we have another one which deals 106 dmg as well. We dual-wield them and we deal a total base dmg of freaking 106 when attking w/ 2 swords. Like...now I think that dual wielding is only useful for Kirito styles....jk xD but srsly tho I think that dual-wield is trash now except it does more dmg when having Sword Skill but w/o that its literally a waste .-. but still.... IMA USE DUAL WIELD FOR MAH WHOLE LIFE XD cuz i just like it xd