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Recent years have witnessed a growing interest in contact-free respiration monitoring leveraging radio-frequency (RF) technologies. However, the proposed solutions mostly consider single-person scenarios, whereas a few multi-person monitoring proposals simply apply blind source separation to handle inter-person interference, without drawing a clear line between physical and algorithmic separability. In this paper, we set out to answer: under what condition(s) one may physically separate multiple respiration signals sensed by diversified RF technologies? Drawing inspiration from conventional signal processing, we propose respiration-to-interference-plus-noise ratio (RINR) as a novel metric, taking into account the impact from both background noise and various interfering sources. Instead of attenuation in Euclidean distance, RINR has to be evaluated upon range/angle bins where physical separation actually …