Fabrication of coupled quantum dot (CQD) may provide an excellent platform for the realization of high end opto-electronic applications as well as quantum information processing. CQD can be synthesized by well-known cation exchange method and less explored ‘nanoparticle fusion’ method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QD) with an emphasis on the formation of CQD comprised of bicompartmental Janus structure (i.e. Janus structure consisting of two compartments with the two QDs coupled) via ‘nanoparticle fusion’. With increase in the ratio of Cd/Zn from 0.9:1 to 1.3:1 to 2.5:1 to 4:1 to 12:1, we observe the evolution of structure from CdZnSeS alloy to Acorn Janus to Bicompartmental Janus_A with ZnS Zinc Blende (ZB) – CdSe Wurtzite (Wz) to Bicompartmental Janus_B (ZnS/CdSe-both ZB) and eventually to CdZnSeS alloy core - CdSe thick shell. Interestingly, the CQD possess two distinct emission bands (570/630 nm) in which 570 nm emission arises from the formation of new electronic states due the strong coupling between the two QDs whereas 630 nm is the characteristic CdSe emission. Further, the coupling enhances the exciton lifetimes of 570/630 nm emission (Bicompartmental Janus_A - 36/31 ns and Bicompartmental Janus_B : 41/94.8 ns) which can be exploited in various applications. Further, the DFT simulations provide the heuristics behind the formation of certain heterostructure with strain and interfacial energies of particular facets dictating the morphology during coupling of QDs.