Yes, some analog enthusiasts have more than 1 tonearm/cartridge combos on their turntables.
Yes, some have 1 tonearm with a stereo cartridge, and another tonearm with a mono cartridge.
Some analog addicts have 2 tonearm/cartridge combos which are both stereo.
The reason for this is that each tonearm and each cartridge have its own characteristic. In order to have more than one grouped (tonearm+cartridge combo) characteristic, the analog addict sets up two sets. Some analog addicts prefer a particular LP to be played in tonearm/cartridge combo A, while another LP is better played using tonearm/cartridge combo B.
Its just like "tube rolling" without the hassle taking-out, installing, and aligning the new tonearm/cartridge combo.
Some tables can even accommodate as many as 3 tonearm/cartridge combos.