Keep in mind the basics of how load works, from p.57:
For each operation, decide what your character's load will be. During the operation, you may say that your character has an item on hand by checking the box
for the item you want to use—up to a number of items equal to your chosen load.
You choose what your load maximum is going to be, which affects how quickly and freely you can move. But you don't actually fill up that load with particular items until you need to use them. It's possible, but unlikely, to start an operation, decide you're heavily loaded, and then never actually bring any items that impact load into play.
So, in addition to the practical impacts of using two pistols or blasters at once:
- bringing more firepower to a single point at once
- getting a greater volume of shots off in any given period of time, if, say, you need to avoid the attention of someone who will come running when they hear gunfire
- threatening a much larger firing arc than you would be able to threaten with one weapon held in one hand
there's the other temptation.
Sure, your single pistol/blaster misfired itself into a piece of scrap, or got knocked out of your hands and down four stories into the drink by some bluecoat/Imperial sharpshooter, or got eaten by an overzealous recycler droid, but that's okay, right? You brought a backup, right? All you need to do is check that little box and pull it out. It's not like you needed that load for anything else, is it?
It's assumed that, being a professional crimesman who is professional at crimes, you can credibly obtain all the stuff on your load list during downtime without even trying. But during an operation, you have what you brought. Did you bring a backup?