Dye visualisation techniques and CFD are employed to examine the flow of viscoplastic Carbopol TM 980 fluids, agitated with dual-impeller systems comprising various combinations of Rushton turbine (RT) and pitched blade turbine (PBT) impellers. The effects of impeller configurations and arrangements on the flows are investigated. Phenomena including mixing, 'cavern'-'cavern' segregation, and flow compartmentalisation are explained by considering the dynamics of the velocity fields and associated flow patterns. Dual-RT agitation induces highly symmetrical flows and dependent on the impeller separation, produces strong, time-dependent flow compartmentalisation. Flow compartmentalisation is overcome in comparable dual-PBT systems due the downward-pumping nature of the PBT producing a state of full-tank homogeneity through a cavern engulfment process. The quality of mixing is compared with mixing effectiveness being maximised for a configuration featuring PBT overlying RT impellers. These results demonstrate clearly the influence of the impeller geometrical arrangement on the mixing effectiveness of viscoplastic fluids in dual-impeller systems.