cant see why he needed duel mag though tbh cant see armed police useing the spray and pray method nor having a prolonged fire fight to require more than 1 mag
Considering that in this country, every bullet fired by the police must be accounted for - as the media would have us believe that the police only ever shoot perfectly innocent, lovely people.
Can you imagine if a police officer fired a three shot burst to take someone down? Think the world would explode from the paperwork. Excessive force and all that jazz.
Anyway, back on topic, if you are going for a look, such as "rebel" or freedom fighter, its fine, or if you have no pockets (one of them there strange naked games), then its necessary. But for the standard military or spec ops look, its not really the done thing.
Still, if you like it, do it, its airsoft.
If you find it a great deal quicker so it gives you an advantage why not - just remember after you have fired both mags, you can't easily slip them into a mag pouch, you would need a dump pouch, which if you were using low caps (i imagine you would be) it would fill up quick.