combining advective reactive transport equation for two solutes

This is a rather basic one sorry, but I am rather stuck.... all I want to do is apply the chain rule to combine the advective transport equation for two solutes such that $\displaystyle dCa/dt =\nabla \cdot (D \nabla Ca)-v \cdot \nabla Ca + \sum Ja $ and $\displaystyle dCb/dt =\nabla \cdot (D \nabla Cb)-v \cdot \nabla Cb + \sum Jb $

where Ca is the concentration of 'a' in the fluid, D is a dispersion coefficient, v is velocity and Ja is the flux of 'a' to the fluid etc

to get $\displaystyle d(Ca/Cb)/dt = ? $ can anyone please help?