Seven isomorphous 2D Co^II_xNi^II_1−x (0 ≤ x ≤ 1) compounds based on ferromagnetic chains with mixed double [(μ-EO-N₃)(μ-N–N-tetrazolate)] (EO = end-on) were synthesized and then structurally and magnetically characterized. They are formulated as [Co^II_xNi^II_1−x(L)(N₃)(H₂O)]·H₂O (1–7), (x = 1 (1), 0.86 (2), 0.74 (3), 0.39 (4), 0.14 (5), 0.10 (6), 0 (7); L = 3-(5-tetrazole)-N-oxide-pyridine). Magnetic studies revealed that the homo-metallic compounds (1 and 7) behave as metamagnets, while the hetero-metallic compounds (2–6) show interesting composition-dependent slow relaxation due to the synergy effect of Co^II and Ni^II ions.