O2 Sensor Replacement Question- Post Cat

[motolz]

I need to replace the O2 sensor on my 1999 996 Carrera. Two questions
1) Is there one or 2 post cat O2 sensors on the car? (are there two cats?)
2) any info on the replacement? I searched and can't seem to find any details. do I need to put it on a lift or can I get to it with jackstands. Any hints or ticks?

Codes are P1121 P0161. both seem to be related to post cat bank 2.

Thanks all. I appreciate the great feedback.