Continuous monitoring of human activity and vital signs has become increasingly important in healthcare and consumer applications. While wearables and others sensors are already in widespread use, they often suffer from issues such as discomfort and the need for physical contact. To address these limitations, wireless sensing using radio signals has emerged as a promising alternative. However, most previous work has focused on bistatic setups utilizing commodity Wi-Fi devices. In this work, we use the open-source openwifi platform for monostatic multi-antenna Wi-Fi sensing for contactless breathing rate measurements. We also present a method for simultaneous estimation of the breathing rate of two targets. Experimental results using CNC machines for emulating human breathing demonstrate the effectiveness of our method and setup for targets at a few meters distance. Even in challenging scenarios where the breathing rates are similar, the targets are close together, and the targets have the same angle of arrival, we achieve an average error of 0.61 breaths per minute. We further validate our setup and method by collecting human breathing data from two subjects, which can clearly be distinguished using our setup and method. Moreover, our results indicate that self-interference cancellation is not necessary for close-range breathing rate sensing.