Unsteady flow dynamics resonated via the natural acoustic mode of a duct with tandem side-branches in a unique half-wavelength arrangement were experimentally investigated. The wavelength corresponded to the first natural acoustic mode of a duct–branch system, which can be easily triggered in a low-speed flow and result in a strong flow–acoustic resonance. Considering the remote arrangement, albeit one involving close flow interactions between the faraway side-branches, a dual particle image velocimetry (PIV) setup was established to simultaneously capture the upstream and downstream side-branches. In addition, an advanced phase-locking technique based on a field-programmable gate array (FPGA) control system and real-time acoustic waveform recognition approach was established to increase the phase-determination accuracy. Using this dual-FPGA-PIV system, the high-frequency acoustically resonated flow dynamics could be accurately divided into multiphase-dependent flow snapshots within an extremely small acoustic period, while ensuring the high spatial resolution of the flow fields. Preliminary pressure measurements and acoustic modal analysis were conducted to confirm the occurrence of flow–acoustic resonance inside such ducts with half-wavelength arranged side-branches. Subsequently, time-averaged flow measurements were obtained using a planar-PIV system to investigate the flow dynamic variations. It was noted that the acoustic modulation effect outweighed the Reynolds number effect and dominated the intensive flow fluctuations in the duct–branch system. Thereafter, phase-dependent flow measurements were obtained using the dual-FPGA-PIV setup to illustrate the spatiotemporal evolutions of the coherent flow structures associated with the resonance in the duct–branch system. Due to the symmetric nature of the resonated first acoustic mode, synchronous in-phase movement of the streamwise shear-layer vortices occurred between the faraway upstream and downstream side-branches. Graphical abstract