The rotationally resolved infrared spectra of (HCOOH)2, (HCOOD)2, and HCOOH-HCOOD complexes have been measured in 7.2 μm region by using a segmented rapid-scan distributed-feedback quantum cascade laser absorption spectrometer to probe a slit supersonic jet expansion. The observed spectra are assigned to the v21 (H-C/O-H in-plane bending) fundamental band of (HCOOH)2, the v15 (H-C/O-D in-plane bending) fundamental band of HCOOH-HCOOD, and the v20 (H-C-O in-plane bending) fundamental band of (HCOOD)2. Strong local perturbations caused by the rotation-tunneling coupling between two tunneling components are observed in (HCOOH)2. The v21 fundamental band of (HCOOH)2 and the previously measured v22 fundamental and v12 + v14 combination bands [K. G. Goroya et al., J. Chem. Phys. 140, 164311 (2014)] are analyzed together, yielding a more precise tunneling splitting in the ground state, 0.011 367(92) cm-1. The band-origin of the v21 band of (HCOOH)2 is 1371.776 74(8) cm-1, and the tunneling splitting decreases to 0.000 38(18) cm-1 upon the vibrational excitation. The vibrational energy is 1386.755 49(16) cm-1 for the v15 vibrational mode of HCOOH-HCOOD and 1391.084 39(17) cm-1 for the v20 vibrational mode of (HCOOD)2. No apparent spectral splittings are resolved for HCOOH-HCOOD and (HCOOD)2 under our experimental conditions. The tunneling splitting in the ground state of HCOOH-HCOOD is estimated to be 0.001 13 cm-1 from its average linewidth.