From the drawings I've attached, you should be able to see that tere is a single fuse for both seat heaters, fuse F15 (20 Amp) on the passenger's compartment fuse board below the steering wheel.
The power feed from there splits off to each seat heater switch and from there to the module under each seat. The power from the heater module is fed to the heater elements, the seat cushion and the seat back or squab are wired in series. The thermostat monitors the seat cushion temperature and controls the activity of the heater module.
Thinking about faults which could affect both seats, they could only be in the "Auxilliary circuits relay", the 20 amp fuse or the ground connection.
Don't disregard the possibility that both seats have gone faulty individually. The areas to consider there would be both heater elements gone faulty, or both thermostats. If you're unlucky then it could even be both heater modules.
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