The transponder control panel is conveniently located on the center pedestal, between each pilot's seat.
Here's a pretty good overall view for orientation:
Transponder is to the right of the center CDU (the thing with the CRT screen and keypad). Above the transponder is the Audio Control panel (mic select, receiver select and volume knobs, etc).
If one guy turned it off (really, to "Standby"), it would turn off the TCAS, but it's not likely to be noticed by the other person immediately...unless they had another airplane obviously in sight, and it wasn't displaying a TCAS symbol.
( I probably should note that the photo is Copyright, owned by Sam Chui - a very interesting travel writer, and great photographer BTW - and is hosted at airliners.net. Page link:
http://www.airliners.net/photo/Emir...91926/L/&sid=450e81f6b88de15699395f124c454051 )