The isothermal crystallization and subsequent melting behavior of isotactic polypropylene (iPP) nucleated with different nucleating agents (NAs) are investigated. Tungsten disulfide (IF-WS2) and N,N'-dicyclohexyl-2,6-naphthalene (NJ) and dual-additive mixtures are introduced into an iPP matrix to generate new materials that exhibit variable alpha- and beta-polymorphism. As shown in previous work, small amounts of IF-WS2 or NJ have a nucleating effect during the crystallization of iPP. However, the isothermal crystallization and melting behavior of iPP nucleated by dual alpha(IF-WS2)/beta(NJ) additive systems are dependent on both the NA composition balance and the crystallization temperature. In particular, our results demonstrate that it is possible to obtain any alpha-phase to beta-phase content ratio by controlling the composition of NAs under appropriate isothermal crystallization conditions. The nucleating behavior of the additives can be illustrated by competitive nucleation, and the correlation between crystallization and melting temperatures and relative alpha- and beta-crystals content in iPP in the nanocomposites is discussed.