What term is used to describe two or more detectors or instruments that are operated simultaneously while located side by side?

The term that describes two or more detectors or instruments operated simultaneously while situated side by side is "collocated." In radon measurement and other scientific contexts, collocation refers to the practice of placing multiple measurement devices in close proximity to allow for direct comparison of their readings under the same environmental conditions. This setup is essential for evaluating the precision and accuracy of the instruments, as it provides a clear understanding of how each device performs in real-time, enabling better assessment of measurement reliability.

In contrast, terms like "duplicated" imply that the instruments are identical and producing repeat measurements, but it does not necessarily specify the simultaneous aspect or proximity. "Calibrated" refers to the process of adjusting instruments to ensure they provide accurate readings but does not pertain to their relative positioning. "Monitored" indicates keeping track of data over time, which does not directly relate to the placement and simultaneous operation of multiple detectors. Therefore, "collocated" is the most accurate term for describing the specific scenario in question.