[CFD_new_user]

I have two cases which are identical to each other in geometry and fluid domain. The simulation is injecting a new fluid into a water domain from two different points. I want to see how this new fluid spreads. I have simulated and have got two result files. Now, separately, I have used isovolumes to check the injected fluid at different concentrations. For example, I have looked at regions with 50 percent new fluid concentration. This I am able to do separately. But I want to see the common region in the domain where the fluid in 50 percent for both the cases (injection points).
Can anybody help me find that out in CFD post?