Passive RFID tag localization has numerous applications, which can benefit from improved accuracy. Ranging methods based on phase difference of arrival become more accurate with higher frequency bandwidth. European regions, following ETSI regulation, historically allowed a single ultra-high frequency band (865–868 MHz) with limited accuracy. Recently allowed, the second band (915–921 MHz) increases the frequency range from 1.8 MHz to 53 MHz, but with a significant gap between the bands. In a dual-band experiment, distance estimates were obtained between two reader antennas and 17 tags across 6 arrays. The results revealed a new 3.07 m ambiguity cycle due to the frequency gap, that adds to the existing 0.17 m phase ambiguity. Solving the new ambiguity on 88% of tag-antenna combinations and 100% of the arrays resulted in a maximum error of 0.55 m (median 0.26 m), significantly improving accuracy (×6.5) compared to the former single-band ranging. Looking ahead, the adoption of dual-band ranging could enhance the accuracy of various tag localization methods in ETSI-regulated regions.