In the present study, we investigate into the symmetry-breaking phenomenon in the wakes produced by two anti-phase pitching foils at zero free-stream velocity. Numerical simula-tions are performed in a gap ratio range of 0.6 ≤ g ≤ 2.0 and a Reynolds number range of 70 ≤ Re ≤ 200. Six regions are identified in the plane (g, Re), based on distinct symmetry-breaking (or symmetry-preserving) behaviours of the wakes. Floquet stability analysis and dynamic mode decomposition (DMD) are performed on some selected cases. By means of modal analysis, the behaviours of wakes in different regions are rationalized and the instability mechanisms of symmetry breaking in the unstable regions are unveiled. In ad-dition, the influence of symmetry-breaking instability on propulsive performance is also discussed.