1) If the temperature sensors are RTDs, you'll need
- some device like an indicator or loop controller that retransmits the temperature signal to the second indicator/controller/PLC
- to change out the sensors to dual element RTDs and use one RTD for one device and the other RTD for the other device.
because you can't parallel an RTD.
2) If the temperature sensors are thermocouples, then you can parallel a thermocouple to 2 different analog inputs; one your PLC, the other an indicator or loop controller
AND IF
- there's no ground loop that creates noise or offset
- the thermocouple break detection circuit on one device does not interfere with the reading on the other device.
Ungrounded thermocouples do this better than do grounded thermocouples.
Sometimes thermocouple break detection can be turned off on one device, but it's there as a safety feature to drive the indicated temperature high which will drive the control output low to prevent overheating/fire danger if the T/C fails.
You'll always have some deviation between the temp reading on device A and device B, but an offset/bias adjustment at operating temperature can minimize that.
Precision Digital (PD) has a T/C scanner (multiplexor), PD138, that cycles through up to eight
T/Cs and displays each temperature in turn, but it does no control, just indication.
http://www.predig.com/PD138/ scanner only
http://www.predig.com/PDS178/index.php scanner and indicator in an enclosure
Although the PD138 multiplexes RTD's too, you'd need dual element RTDs, one for the scanner, one for the PLC.
