> Low strength requirement
Your dual beretta soldier will be able to wear more utilities like grenades, smokes, sensor grenades, medikits, stun rods, motion sensors... But with two hands full, he will be unable to quickly use any of them.
It's easy to holster (or drop) one if a grenade is needed, and it's a tradeoff for the extra firepower you get.
> low TU cost for individual weapon fire if needed,
You already get this by equipping a pistol in one hand and a two-handed weapon in the other; or keeping one hand free and a pistol in belt (2TU to draw)
4 to draw, actually, at least by original X-Com rules. And 8 to put back, otherwise you lose accuracy on the main weapon next turn.
> double the ammo capacity between reloads,
Well no : If you compare akimbo shots with the single pistol, you reload after as many Snap(x2). If you compare akimbo shots with rifles/cannons, you reload even more often, because rifle clips are larger.
But for single pistol shots, you have twice as much ammo. And considering that the Pistol isn't that far behind the Rifle for damage, you could actually deal as much as 160% the damage of a Rifle shot with every 2xSnap, and you only carry 40% less ammo for that.
Let's run some numbers here. Let's say a 2xSnap with Pistols costs 25%TU, up from 18%, and accuracy degrades to 45% from 60%, but you fire two 26 AP shots. A Rifle gives you a single 60%, 30 AP shot, for the same 25% TU. Or it gives you three 35%, 30 AP shots, for 35% TU. The dual Pistols would actually be at a rather decent advantage, damage-per-turn-wise, although you pay for it with some logistical problems, as outlined above.
edit: Anyway, I find it looks silly if soldiers that have access to fighter jets, tanks and rocket launchers choose to fight instead with ... twin colt 45s.
.45 ACP is a pretty popular, and very powerful round, I should say. If any soldier were to be capable enough to use two .45 handguns simultaneously (in game terns, has high TU and Accuracy), I wouldn't be surprised if he would do so. And this does extend past mere pistols - laser pistols and plasma pistols come later, as well as any modded single-handed SMGs with alloy rounds.