In reactive inkjet printing, two inkjet printheads are used to deposit droplets of different fluids which coalesce and react on a substrate to form the desired product in situ. In this way, inkjet printing can be extended to produce a wider range of products, which may be challenging to jet directly. Whilst there have been many successful examples of reactive inkjet printing, such as biocompatible tissue scaffolds and micron-scale polyurethane structures, efficient mixing between the coalescing droplets is a requirement for the desired chemical reaction to occur. Previous work has demonstrated surprisingly little advective mixing between impacting and coalescing droplets of the same fluid, with dynamical dimensionless numbers matched to values common in inkjet printing. This work studies the internal dynamics of coalescing droplets, of like and different fluids, in contact with a solid substrate. Experiments utilising high speed cameras, and numerical simulations in OpenFOAM, are used to capture the internal dynamics and associated mixing. Using a variety of initial conditions, different aspects influencing the internal dynamics are investigated, including a negligible initial velocity case to focus on the influence of substrate wettability and an impacting case with clear relevance to reactive inkjet printing. Key factors affecting the internal dynamics are investigated, including droplet volume ratio, impact velocity and droplet composition, with approaches to improve advective mixing identified.