GreasyDogMeat: Pressing '2' a second time should switch. If you are using keys or mouse wheel for 'next' & 'previous weapon' it will also appear after the single magnum/shotgun. Also pressing your alternate weapon button, the same one you would press if you had a sniper rifle and wanted to zoom the scope, will switch between single and dual magnums. Also, of course, make sure you have actually found and picked up a second magnum. Blade has a habit of starting many levels with only 1 magnum needing to find a second.
Thought I was losing it for a second, but apparently this only works in Wages of Sin? I've been playing the base game so far and can't get it to work there (even tried '/spawn Magnum' to make sure). In Wages it works just as you described though.
*edit: sorry; just noticed the 'by launching it through the expansion' bit in your first reply :S
So thanks for the info, all sorted out now; my memory was playing tricks on me :)