Good advice all around.
If you can find a company making a hand that's similar to the model's other hand and holding the kind of gun you want to use then that's a great way to go. In addition to JB's suggestions, Reaper also has some nice hands holding pistols.
Otherwise I just remove the handle from the pistol you want to add. Then clip away the pistol you don't want, file it smooth on top and attach.
If working with metal, it's best to use a different clippers than the ones you use for plastic. You can usually find a cheap pair of "Flush" clippers for not much $ at all. Metal will quickly make a set of clippers bad for plastics. Also, if using metal, I prefer a rubberized superglue such as Gorilla "impact Tough" or one of the BSI rubberized glues (Instaflex or IC-2000) as they give a bit more flex and aren't as brittle as other cyanoacrylates.
If you are using metal and want to be really sure the bond is tight, drill a hole in the top of the hand, and then clip the handle of the gun you want to use into a peg. Then glue together. It's basically pinning without a pin.