Chemical imaging applications using Laser Induced Breakdown Spectroscopy (LIBS) involve laser ablation sampling at interfaces or boundaries between materials with different thermophysical and optical properties, leading to the formation of two initially isolated plasmas. The mechanisms of mixing in these plasmas and their direct implications on spectral emission and chemical imaging remain largely unknown. We investigate the mixing dynamics of isolated plasmas produced using femtosecond (fs) laser pulses at the interface between two different model Cu and Sn-Pb samples. Specifically, we study the spatial and temporal mixing dynamics and outer plasma expansion dynamics of the fs plasma components. The time-resolved degree of component mixing, horizontal and vertical expansion, mixing speed, and temperature and electron density of the heterogeneous plasma are characterized. Mixing processes are initiated early on, less that 100 nanoseconds after the laser pulse, and continue taking place until the end of plasma emission persistence at a constant velocity of 104 cm/s. These findings demonstrate that proper selection of detection timing parameters and signal collection position are critically important in precise LIBS measurements to ensure that full plasma mixing has taken place and that homogeneous spectral emission is properly detected.