A theoretical model has been derived for the calculation of small-angle X-ray scattering (SAXS) data from stacks of crystalline lamellae in which two or more kinds of lamellae are mixed within the same stack according to different statistical models, ranging from the fully random mode, to alternated one, up to block configurations. The model has been used to study the complex lamellar morphology which develops from melt in some blends obtained by mixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoreoregularity, similar molecular masses, and melting temperatures of approximate to 162 degrees C for the more stereoregular component iPP1 and approximate to 74 degrees C for the less stereoregular one iPP2. It is shown that for samples crystallized in conditions far from thermodynamic equilibrium, the observed SAXS intensity profiles may easily show absence of correlation peaks, associated with high amount of diffuse scattering, not easy to unravel. Using our theoretical approach we show that featureless SAXS intensity distribution may correspond to formation of disordered lamellar stacks with lamellae of the two components randomly mixed within the same stacks. The tendency of iPP1 and iPP2 to form mixed lamellar stacks with a random configuration provides clear evidence that the two components are miscible in the melt.