Can't seem to get an answer either. On the petrols there's two one at the top of rad and one at the back of the engine. If you have two one will be where I showed you on that pic and the other bear the rad would be my guess. My advice would be for you just to ring a parts department, give them your vin and they'll be able to sort you out with the parts. Otherwise you're running the risk of getting the wrong parts. Or speak to Audi.
Here's the info on two sensors:
"There are two temperature sensors in some vehicles, one to send information from the engine system to the control unit and another from the control unit to the dashboard. The device follows the principle of dependence of potential difference in temperature."