The BEST 2 gun technique is having a backup pistol in addition to your main weapon. That way if your main weapon malfunctions, runs out of ammo, gets taken or dropped you have another to immediately go to. Whenever possible any pistol you shoot should be held 2 handed for maximum stability, accuracy and control. Trying to shoot 2 guns at once, in the real world, puts you in a giant trick bag of shit as far as liability goes. There is NO WAY you can accurately aim 2 guns at once and even if you just shoot one at a time there is still a second gun hanging out there which can still be fired, even inadvertently, inaccurately. It is a highly unsteady platform to shoot from with a gun in both hands. Looks good with Rambo, but not with Joe Blow's like us.
The only other 2 gun system I could think of worth a damn would be 2 weapons mounted side by side. Would need a SINGLE sighting system and an ability to mount it steady such as a tripod or even a bipod and a single firing system. Not practical for any civilian or even police uses.
However, I would LOVE to have a Quad MG42 setup in the bed of a Chevy pickup. Now THAT would be the cats meow!! Could call it the Democrat Reeducation System.