Assuming it's similar to the 40v S6, the post at lambdas are below the handbrake but the pre-cat ones are in the exhaust downpipe close to the cylinder head.
The pre-cat lambdas aren't easy to get to in-situ, there might well be a special tool to get to them but I've not got it, the post cat ones are fairly easy to get to though. Personally, if I was swapping the pre-cat sensors, I'd drop the down pipe off the car.
This is how I did it on my S6, S8 should be broadly similar??
http://forum.a8parts.co.uk/showthread.php?t=4834