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From the Raspberry Pi Foundation blog post describing this add-on: https://www.raspberrypi.org/blog/introd ... t-poe-hat/
Because the fan is controlled over I2C, none of the GPIO are used, so you can stack a second HAT on top of the connector. To do this you will need to buy some longer pass-through headers that expose the pins on the other side of the PoE HAT. You will need one for the 40-way and one for the 4-way connector that has the PoE splitters on it.
Edit: beaten to the Submit button, but left here for reinforcement, not as a deliberate me-too echo...