Transmitter max output when checking ohms in parallel
An ohmmeter applies a current to the component under test, and measures the resulting voltage (or applies a voltage and measures the current).  Other components in parallel with the the component being measured, or voltage or current appied by another circuit, will make the ohmmeter measurement invalid.

The RTD measurement circuit will also apply a voltage or current to the RTD, so the two measurements will interfere with each other, and any readings you obtain from either the ohmmeter or the RTD measurement circuit will be invalid.