The problem is further overblowing power at the sacrifice of defense.....
Many here already know I am vehemently against Chaos and Black Kat sets..... Adding a weapon mechanic on par with these blights of armors would probably make me stop playing all together..... if not just go and fully withdraw from playing in groups.......
Besides we already have switch shooting which is more defensive and efficient than your proposal, just takes more skill to do...
Dual wielding has been suggested many times, and all it really should be is a cosmetic appearance on a new weapon line.
Shields are also very critical to the game. All advanced combat tactics require using the shield for manipulation of animations and whatnot.
Also remember, not everyone uses the mouse for attack/shield. That's just one of the control schemes.
I use a hybrid keyboard mouse layout in which I attack with mouse2 and I walk with mouse1.
Well, if it WERE made I would say swords and bombs could dual wield as well. Which makes a first no. If it were just guns, everyone would be buying guns, making no swords-men, and swords are EXTREMELY important. But if OTHER weapons could do it to, dual swords and bombs would become powerfully OP. (Especially bombs.)
But, another -1 is, Hall of Heroes. I think that having 2 weapons instead of a weapon and shield to pass Soldier, Knight, Knight Elite and Defender Elite Hall of Heroes (So just Hall of Heroes in general) is a bit... Not that great, I prefer just 1 weapon and 1 shield.
And dual wielding would make either everyone die, or everyone OP. So perhaps not. Sorry. I DO think other than that, it sounds pretty cool. I like the idea of dual wielding, just not sure how it would fit in the game...
*lists many others who suggested this idea*