I really do not understand why to be quite hostile and as you self said so: "frank". We all agree, even if my idea might be stupid, I still invested some time in writing that stuff and I do that of course not becaause I try to make OC worse. Anyway, lets just get back to "neutral"

You acutally argue again, dual wielding system would be more complicated than the current system. Which I say it does not have to.
So my idea (~approach) i imagine as follows:
Inventory consists 5 items. however the fifth item might become the second hand slot. You still should be able to switch between the items by mousewheel or numbers. Furthermore you get an additional slot number 6, which becomes the righthand slot. assigning an item could simply by drag and drop, or some combination like ctrl+6, that swiches the selected item to that passive right-hand- slot. The Item in this slot is used by rightclick. This 6 item cannot accessed by mousewheel anymore (which is of course not needed because it is allways right-click selected)
As rightclick is reserved than for that right-hand slot (which is rather unflexible in terms of Item chance, compared to the left-click item), throwing an item has to work differently now. Lets get back to the roots. In CR thowing items was done by pressing the "throw"-key. As that is, as also ala and you mentioned in clonk rather important, that could be the easily accessible Q button. This kind of way of control is actually currently used in the Knueppeln-game play, where shooting the main attack is done by Q, instead by klicking arround, and still feels very natural. So that could just be adapted to throwing items now, which would reduce the confusion with right-click throw and left-click use.(of which i find really weird [but that really is very subjective])
I argue that this system is as simple as the current system, with no chance of mistakenly forget about which hand has which. The confusion is avoided because the right-hand item is rather fixed and set usually to shield or whatever. (however it still can be changed rather quickly, if neccessary).  You usually actively use still only the left-hand item. Well and eventually one standard item slot should be blocked if something is put into the right-hand slot (otherwise you end up with an inventory that can hold up to 6 items)

That approach would:
-bring back the coolness of wielding sword and shield, or any other items simultaneously.
-would avoid confusion with right and left click use and throw (which does not only appear to me)
-would still allow simple and quick item throwing just by a the Q key instead of right click (which is not different to CR, or casting the standard spell in knüppeln)
-would get rid of that funny quickchange, which may be handy, but needs some experience to efficiently use.
-keep the item switching and using as comfortable, simple and easy as it is now.

Oh and I almost forgot: the funny argumentation of that youtube guy is somewhat not applyable to OC or other games because:
he mainly argues a shield is much more sensible than a second weapon, which i definitly agree in real life. One arrow or one sword strike may mean: you are dead. In OC (and any other games it just means not having a shield makes you more vulnerable, but you still will not die immediatly).
The more striking argument for game situation is the attack-speed one. He claims, you can eigher use your whole body and increase the strenght of one strike (which leaves the other wielded hand in a position where you anyway cannot usefully attack with it) or you can just swing your weapon as quickly with one hand as you can do with both. But that is not entirely true. You can simply not do both with just one weapon: you cannot use your whole body and make quick full power stikes. Especially  having some weights in your hand (which that bloke does not). Just try it out with holding a filled two liter bottle and try to swing it quickly. That is not easily possible. You can however use your whole body striking powerfully and while wielding two weapons ohne arm charges while the other one strikes. So effectivle you are almost as twice as quick in striking that just with charge and strike with only one hand. Anyway, it surely leaves you without any protection or possibility of parade.
But acutally that kind of dual wielded movement is what kids do when they are in rage and try to box you with two fists. And also boxers do likewise sometimes (you can definitly not box with one hand as quick as with two, because that allways needs some body turning for charging).  Here the same argument applies, boxers do not die immediatly loose if they use both hands for attack, however it may leave them more vulnerable)