A numerical investigation was carried out to analyze the interactions between semi-active tandem flapping foils at various tandem distances with a chord-based Reynolds number of 1100. Results indicate that with a tandem distance of less than 1.5 chord lengths and released in-phase, both foils exhibited terminal periodic motions with a nonzero mean stagger distance. In contrast, under the other conditions, the two foils ended up with periodic flapping motions without stagger. Due to the high-pressure region near the leading edge of the aft foil, the heaving motion of the fore foil resulted in lower energy extraction performance than that of single foil, when the tandem distance was less than 5 chord lengths. However, as the tandem distance increased, the fore foil acted like a single foil. The aft foil demonstrated significant fluctuations in performance parameters when subjected to the wake of the fore foil. The favorable interaction between the wake and aft foil resulted in lower power consumption for pitching and enabled the aft foil to extract an additional 15.2% power compared to a single foil. Conversely, during the unfavorable wake–foil interaction, the pitching motion of the aft foil consumed more energy than energy extraction from the heaving motion, leading to net energy consumption. The initial inter-foil pitching phase difference also significantly influenced the performance of the aft foil. Two models, the global phase and the wake phase model, affect these tandem configurations, both proving effective in capturing these effects with the wake phase model displaying notable efficacy.