In ultra-low power radios, envelope detectors are often used in the receivers to convert the RF signal down to the baseband [1-3]. However, such radios suffer from poor frequency selectivity. In this work, an interference suppression technique is proposed, which makes use of 2 RF tones to carry information. It introduces frequency selectivity to envelope detector based transceivers, while maintaining their simplicity and low power consumption. System level simulations verify that the technique not only improves the in-band interference rejection, but also allows coexistence among multiple users.