One turntable with one arm.
But my arm is the Simon Yorke Aeroarm that is an air-bearing parallel tracker. Alternative moving carriages (arms) can each be ptr-set up with a cartridge and swapped out in a couple of minutes - only need to slide the carriage on the bearing rod and disconnect and reconnect air-line and signal plug.
Best of both worlds, and parallel tracking too. State of the art for ultra low-mass MC carts up to 10g such as vdH Colibri and Ortofon A95, both 6g.