Need to know,
regarding setting the code on the other transponder, and then changing over.
It depends on avionics fit. Even in an aircraft with two tranponders, it may not be possible to set the code on the transponder not in use. Although you may have 2 tranponder boxes in the rack, often they are operated by a single control head. This would have a toggle switch to change between 1 and 2, but only one set of code setting knobs. Does that make sense?
I.E. you can't change the code on the box not in use.
If I can just reiterate for those who are clinging to the 'Standby' method, please don't do it. Even if your transponder is at standby for say 5 seconds that is still a large proportion of the alert time thresholds for TCAS at low levels. If you are really worried about inadvertantly selecting an emergency code, it pretty easy to avoid going via 77, 76, or 7500. You're much more likely to get caught out by the single push 'squawk emergency' or memory features that some control heads have.
CPB