Kerry Wong has designed and built a dual sensor fan controller:
To make the design more useful, one channel is marked as optional (see components with * on the silkscreen) so that you can build either a single sensor fan controller or a dual sensor one. With two sensors, the control signal is or’d with both of the sensor input and if either side of the temperature exceeds a predefined threshold the fan will be turned on. This is useful in situations where power devices from multiple channels share the same heat sink
Project info at Kerry Wong’s blog.