Polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends filled with surface treated graphene have been fabricated to obtain conductive composites with a lower electrical percolation threshold according to the concept of double percolation. Obviously optimum morphology for this blend is co-continuous that represent the lowest percolation-threshold in compare with other morphologies. In this study co-continuity window of the mentioned blend has calculated theoretically by use of rheological models and then verified with experimental results comprise of microscopic pictures and RMS curves. In addition to determining co-continuity window of this blend, effect of adding surface treated graphene to it is studied and the relation between grephene content and shift in co-continuity window is achieved. Rheological and electrical properties of the blend was investigated and electrical percolation-threshold of it at the top boundary of co-continuity window of immiscible blend is calculated (0.3 wt %) and compared with result of other researchers who disregard the volume fraction of immiscible blend with co-continuous morphology (Mao et al.). Electromagnetic interference shielding effectiveness (EMI-SE) of the blend is also studied and results shows a significant growth in EMI-SE of the blend in compare with similar blends having same electrical conductive filler content. This study represents that change in volume fraction of immiscible constituents of the blend in co-continuity confine has significant effect on electrical conductive network of particles and percolation-threshold of it. Change in this confinement due to adding surface treated nano-particles is also represented.