This idea has been suggest lot time, Just wonder how is it going on this system.
As with most other things, it’s current label is: “Waiting until somebody decides to work on it”. 
There are many, many ideas out there, and many of them are awesome things to be implemented, but there is only a very tiny handful of devs, and as such the vast majority of those awesome things are going to be stuck in a limbo until someone gets around to them.
To be a tiny bit more specific, since that particular code deals with the inventory code (which is kinda dense), but otherwise doesn’t really have too many things that it would need to change, I’d probably put it in the “could be done by a focuses coder working on it” category. It’s definitely not something that someone is gonna crank out in an hour or two, but it’s nowhere near as much work as some of the bigger features like Z-levels are.
Also to manage expectations, I have a very poor opinion of the concept of dual-wielding most guns and melee weapons, so things like wielding two swords or two pistols is generally going to be a very bad idea.
There are several other cases such as shield use and wielding two very light weapons where it makes a lot of sense.
Maybe adding shield as (W)earable option aka clothing with varying coverages?
Until someone codes a good way to execute blocks those would be useless.
Consider currently existing arm guard items: due to their coverage around 30%, you can’t arm-block with those, just with the whole arm.
So the shield would have to cover both arms with coverage near 100% just to be used in arm blocks.
Scaling item coverage on limbs used to blocks would probably be enough to make shields (and arm guards) useful.
If someone came up some clean code that would allow wearing a shield to properly encumber the wielder and interact properly with blocking, it’d go in. I’m personally not interested in implementing a half-measure, I’d prefer to add tracking for a second limb and all that entails, but it’s also not high on my list of priorities.
Yeah, I think tracking a second limb is probably gonna be the best way to do this. I’ve actually looked into it and it’s kinda close to the top (not all of the way at the top, but it’s one of the ideas I’ve been sort of looking into). I’m not gonna throw out a timeframe, since who knows if another awesome idea will pop up first, but it’s definitely one of the things that I’m currently interested in.
Would briefcases be some kind of improvised shield, then?