Non-premixed syngas combustion anchored by a bluff body is investigated for its multimodal operation of low- and high-frequency instability corresponding to different Reynolds numbers. Simultaneous high-speed imaging of OH* and CO2* chemiluminescence is exploited with syngas to examine synchronous or sequential twin heat release rate zones from H2 oxidation to OH and CO oxidation by OH, based on past studies from this group. Flame dynamics of both low- and high-frequency oscillations reveals that the former is significantly dictated by flame residing in the shear layer and in the wall vicinity, whereas with the latter, it has both lateral and the aforementioned stabilization contributing in equal measure. By performing a causality-based synchronization measure, we determine regions of synchronization and the degree of causality between the two heat release rate zones. It is seen that the high-amplitude low-frequency oscillations have lesser but near instantaneously interacting synchronous regions. High-frequency oscillations display multi-valued causal indices that illustrate the existence of lag synchronization. Further, low-frequency oscillations display dispersion in peaks of chemiluminescence, resulting in loss of synchronization in downstream regions. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.