If the lights in the switches light up ok then the fuse is fine anyway.
I can't find anyway that one seat can have any effect on the other seat. They both have seperate supplies from the switch, and both the seats, and the switch illuminations have different earth points. So unless there are a load of earths not connected, I can't see any connection at all from one side to the other. Each seat seems to have two halves to the heater circuit. A thermostat controls the overall temp of the whole seat, and a second controls the lower temp range. I assume it is like a safety feature, making sure if the temp goes too high, the whole heater is switched off. If the seat back harnes is not connected to the seat base harness, then that would stop the heater completely, as the circuit goes through the base, then the back and then to earth.
Are you sure they are not working? Did you feel the other seats working before removal from the other car? On my mk1, it seemed to take forever for the seats to heat up, on my mk2, they are much, much faster, almost instant.
Have you tested continuity through the seats?
On the seat base plug,
At seat temp below 30°C, there should be continuity between pin 1 and 3.
At seat temp below 30°C, there should be continuity between pin 2 and 3.
At seat temp above 50°C, there should be no continuity between pin 1 and 3.
At seat temp above 40°C, there should be continuity between pin 2 and 3.
On the seat back,
pins 1 and 3 should have constant continuity
pins 2 and 3 should have constant continuity
Pin 3 to earth should have constant continuity
Other than that, I can't think what else to check.