The hydrocarbon parts of (C10H21NH3)2CdCl4 and (C10H21NH3)2CuCl4 represent smectic lipid bi-layers exhibiting two structural phase transitions analogous to those in biomembranes. A¹³C nuclear magnetic resonance study showed that in the Cd compound the low-temperature transitions is connected with a disordering of the polar heads whereas the high-temperature transition corresponds to a partial melting of the alkyl chains. In the Cu compound the sequence of the two successive transitions is reversed. A Landau theory describing the two transitions in terms of order parameters used in the theory of smectic liquid crystals is presented.