I don't know what you were expecting out of the pint sized mini rig! Mathematically a 2nd cab is going to add a little and then some, but it's still only going to be 2 tens at the end of the day.
Afaik Elf manages about 130w into 8 ohm, so not a whole lot of extra power, but the 3dB of extra sensitivity from doubling cabs can do a bit extra with it again. If you are seriously only lacking a little then I feel you would do nicely with the 2nd cab.
If you are more underwhelmed than that you might be better off with a 12 or two.
Never mind what other folks get away with using with their loud drummer. Everyone's drummer is loud. Some are louder. If you ever plugged in a second cab you should have a fair idea what to expect in your situation.