MIcro-mixing of two different fluids

Hello,

In micro-fluid domain, fluids (which don't react with each other) may mix only by diffusion. So if I have two different fluids, having different inlets and a common outlet (lets assume a T- junction), with diffusion coefficients given, how would I go about simulating it ? I get stuck at how would I define such a domain and how would I couple the two physics. I am a novice.

Thank you for your help and time !