Sorry for the necro but, I wanted to provide my 2 cents on this.
First, one critical use I would use akimbo for, is allowing multiple target firing with less tu usage compared to one. Imagine 2 smgs fired in opposite directions. You could not only double the fire rate, but target more enemies in a turn this way.
I'm no mathematician but I thought some about possible balance for it.
first, if 2 firearms are in hand, give some sort of option for akimbo combined, or independent.
Combined would be roughly as explained above, firing both at the same target. I wouldn't explicitly lock out 2 handed weapons that can function 2 handed, I'd just give it MISERABLE accuracy, and less efficiency in doing so compared to hand weapons. But I wouldn't really lock out using multiple types of guns together, I'd just round to the largest shot value for both. Not ideal, but would be easy to implement.
Independent would be a bit janky to use, but would give much more versatility. my idea is you could enable akimbo independent stance in the weapon choices, which works like a toggle, with possibly a minimal 2 tu cost, or possibly none at all, and it would work like the following : first off, you have slightly lowered accuracy, possibly more than what dual fire would be (since you concentrate on 2 guns), and if you fire one, it discounts firing the other by 90% the cost of the first, yet the second is less accurate, with extra penalty for amount you must turn. (shooting behind you would be a crap shot if you used this). After the second hand is fired, it procs back to akimbo ready, allowing you to fire the first gun again, OR, disable akimbo to fire more accurately. This would enable true akimbo, rather than effectively strapping 2 pistols together. you could, with a good troop, make pistols much more effective. However akimbo cancels if you move, to prevent unrealistic abuse (fire one gun, run 4 tiles, fire again for free?)
I was also thinking bringing reactions into this, I'm not certain of the math, but perhaps besides the slight drop in accuracy, subtract reactions from firing skill, and whatever is left over you perhaps multiply (.5?) and then also subtract from aim. Also, penalty on turning could also be effected by reactions, the same way. This way if you are an amazing shot, BUT, a terrible reactions person, akimbo will be far worse, but a mediocre shot with high reactions with them will at least be more stable over all. Perhaps akimbo could also effect reactions chance slightly so akimbo doesn't trigger reactions twice as much. As a balance issue, akimbo could also WIDEN the possible cone of fire on said firearms to emulate awkwardness of firing 2 guns at once and trying to aim.
I know independent fire would be a much more complex coding to do, most certainly, but being able to fire them independently would make for a VERY interesting class dynamic....Imagine a magnum with smg combo, in decent hands, shooting a guy at 10 units, and spraying a guy to his side in half the time as normal. It would also allow particularly skilled troops to engage twice as many enemies, which, even with the accuracy issue, I could see double uzis being extremely useful in CQC engagements.
Honestly, I'd go with JUST independent fire, where you could effectively combine them IF YOU WANTED, but I figured I'd mention both. I know its more involved to code, and use, but the versatility added to it is well worth it in my opinion. And even with bad accuracy, this, and spray and pray feature, would give me a reason to try double uzis.