That’s why I thought it was that they needed two mics as they didn’t have a splitter. Or possibly they had the second mic running to a processor, like a Leslie cabinet for another song.
I don’t know if this is the case here, but as some mikes are better for certain kinds of vocals than others. For example a ribbon mike is great for light vocals, but would be blown by a rock singer. Since some singers do both light vocals and rock, they would turn off the one not suited to the song.