I'm thinking about using two different RTD sensors, one in my RIMS tube to monitor the mash them then another in an instrument T to monitor chill temp. Is it possible to swap the cord using the same PID, or do they need to be calibrated?
The PID 'learns' the profile of the vessel the RTD is in. If you switch back and forth on the same PID it will never learn the system and you'll never hit temps as well as you would without swapping. But it can be done.
The PID should allow you to specify a temp offset to calibrate the temp and account for the sensor cabling and internal circuitry. I think the two RTDs probes will be closer if they have the same cable length.
You can probably calibrate for the mash sensor and +/- a degree is not critical for the chill temp
if the 2 sensors read different for the same thing then just note the difference and set the offset for the Mash rtd. Then when you are chilling just know you have to add or subract X number of degrees for the actual temp.