As far as I know, Coulomb's law of electrostatic force is applicable on two different charges situated in same medium.
Only in fluids (gas, liquid). In solids, Coulomb's law cannot be directly applied because the solid contributes unknown amount of force itself (it maintains mechanical stress). One needs to carefully analyze forces in solids; usually people use stress tensor.
But if two individual charges are in different media (say one charge on a iron surface another on a plastic surface), then how does one calculate the force between them?
If the bodies with charges are separated by vacuum or a fluid (air, water), you can use Coulomb's formula with $\epsilon$ of the medium.