The 212R has two 8 ohm, 50W, 12 inch SD speakers in parallel, resulting in a 4 ohm total impedance. So yes, if you unplug one of them, it will deliver half the output sound power.
The only thing I don't understand is why would you want that - can't you use the volume knob to do that?
it only sounds real good if you crank the master volume up and then the overall sound is just too loud for the stage... I really would like a power soak/attenuator to achieve this. But dunno how to build on really...see? the amp sounds crap with the master vol down low.. Am wondering, if I did detach one of the speakers would it not damage the output transformer?