IvanMike is right. Again, Kustom isn't a high end brand- not even middle of the road, actually- and chances are the power supply and output devices of the amp aren't capable of driving the high current load that adding a second cabinet in parallel with the internal speaker would present. Switching the internal speaker out of circuit is Kustom's way of keeping you from burning up the amp.
Getting more volume isn't merely a matter of adding more speakers, it's a matter of using speakers with better frequency response and higher sensitivity than what you already have. The way to find out what will work, aside from taking a course or two in audio engineering, is to take your amp to a music store and try it with other speakers. Going the mail order route when you don't have the requisite expertise to know what you're getting based on specs alone isn't a good idea.
