Let me explain the scenario. You have an electric guitar player with a combination amplifier (combo) that has two identical loudspeaker drive units.
You can put the mic on the left drive unit, or on the right drive unit. Which do you choose?
I suppose I should also mention the possibility of placing the mic exactly dead central. You can do this, although the mic would perhaps have to be more distant than if you miked just one of the drive units. There isn't really a great deal of point in doing this, although you are welcome to try.
So left or right?
Firstly, do the drive units sound different? In a new or newish combo, then they should both sound exactly the same. But often they don't, so it is worth experimenting with the microphone to find out which is best. This is something that can't really be judged by ear since the drive units are physically so close and you would damage your hearing if you put your ear right up to the cabinet.
But if they do sound the same, then the mic can go on either side surely?
Wrong. Take a look round the back and see which side the mains cable goes in. The mic goes on the opposite side.
The reason for this is that dynamic microphones are sensitive to magnetic fields. The mic will easily pick up the 50 Hz or 60 Hz mains hum from the transformer. Placing the microphone on the other side of the cabinet, thus distancing it from the transformer, will lower the hum level significantly.
Of course, some microphones are less sensitive to hum than others. And if you use a capacitor microphone, it won't pick up magnetically radiated hum at all. Capacitor microphones work using electrostatic fields and are not affected by magnetism.