Yes, the diagram in the manual shows two amps. That is for stereo operation and as you say, your Mc100 is not stereo. It is one channel mono only, but as I stated above, the manual does not warn or say not to use more than one impedance simultaneously. The manual does mention that mismatching does not compromise any built in protection and is no problem except for distortion at high volumes.
Yes, wiring the two cabs in series and connecting them to a single MC100 connection (ie: MC100 -> cab1 -> cab2) will work, no problem. You would have about 6 ohms all together and could come right off the 4 ohm tap with no worries. I would certainly do it this way if you are concerned about using simultaneous impedances.
First show: 1973
Last show: 1994