Only having either the silenced or normal SMG can get rather annoying, since you need one or the other for certain situations. There is a way though, to have both of the Sub Machine Guns. First, you must complete a level with the SMG in it (SMG is first seen in Japan), and go back to your garden shed. From there, go to the tool shed, grab your weapons, and make sure to include the SMG gun and the Katana (Sword). On the next level, if the silenced SMG is in it, then you are in luck. Never actually use any of the 2 handed un-concealable weapons until you find the silenced SMG. When you find it, pick it up. The weapon in your hands (Even though the sword is awesome, the SMG with silencer can get you out of most binds) will fall to the ground, and the new SMG will go in it.

Tada! Now you have an SMG in your hands, and an SMG in your inventory!