We probe the effect of heteroatom substitution on the spin crossover (SCO) properties of dinuclear materials of the type [Fe₂(NCX)₄(R-trz)₅]·S (X = S, Se; S = solvent; R-trz = (E)-N-(furan-2-ylmethylene)-4H-1,2,4-triazol-4-amine (furtrz); (E)-N-(thiophen-2-ylmethylene)-4H-1,2,4-triazole-4-amine (thtrz)). For the furtrz family ([Fe₂(NCX)₄(furtrz)₅]·furtrz·MeOH; X = S (furtrz-S) and X = Se (furtrz-Se)) gradual and incomplete one-step SCO transitions are observed (furtrz-S (T_1/2 = 172 K) and furtrz-Se (T_1/2 = 205 K)) and a structural evolution from [HS–HS] to [HS–LS] per dinuclear species. Contrasting this, within the thtrz family ([Fe₂(NCX)₄(thtrz)₅]·4MeOH; X = S (thtrz-S) and X = Se (thtrz-Se)) more varied SCO transitions are observed, with thtrz-S being SCO-inactive (high spin) and thtrz-Se showing a rare complete two-step SCO transition (T_1/2(1,2) = 170, 200 K) in which the Fe^II sites transition from [HS–HS] to [HS–LS] to [LS–LS] per dinuclear unit with no long range ordering of spin-states at the intermediate plateau. Detailed structure–function analyses have been conducted within this growing dinuclear family to rationalise these diverse spin-switching properties.