Nio
What do you mean by Alternate fire ?
In other words, don't squeeze both triggers at the same time. Switch between guns. Fire RIGHT-LEFT-RIGHT-LEFT. Literally switch between guns. Sight right, fire, sight left, fire. You train both hands that way. In order to train, start this slowly. Maybe a full two seconds between shots at the range. Then start working faster. It's best to start this with dry fire practice first. (Or with AirSoft, which is what I did early on.) Once you get the hang of that, you can easily do half second splits between the guns. Once you get good at it, your split time gets really quick.
Alternatively, you can train to fire both guns at the same target at the same time. An instant double tap... I don't really like that as much, although it is easier.
I always conclude my practice by firing as fast as I possibly can while still maintaining some form of accuracy. There are times when throwing as much lead away from you as fast as you can is a good thing. Like when zombies attack or something.
Of course, most of the time you aren't going to do that in real life. You are responsible for misses, and misses don't help you stop bad guys.
Is it impossible to change the lenght your arms are at in relation to one another while shooting
No it isn't. Just twist your torso. The problem with that, of course, is that one of the guns gets closer to your face, and you end up shooting 'past' the other hand - which means that in a high stress encounter you could shoot your own hand. On the good-bad scale, that's really bad.
What I like to do in my practice now is a nearly 180 degree spread between multiple targets. I fire one, spin my head to the other, fire two, spin my head back. You get the idea. It's side-side dynamic shooting. Then I add movement. I'm fortunate to have access to a 360 live fire range on private property, though. I imagine that it would be very hard to train like that for most people. That's why I reccomend airsoft to start. It's cheap, you can try crazy stuff, and you won't kill yourself while finding out what works and what doesn't FOR YOU.
Never listen to guys that say, 'You can't do that.' or 'That doesn't work.' YOU try it and decide for yourself. I do some strange things that work for me that my instructor goes crazy over. I won't do a 2 hand hold, for instance. My chest is too big and I feel constrained. Do what works for you.
Ed McGivern's book will give you some ideas of what is possible. Best advice, though, is to pick a style and stick with it.
Nio