Problem assignment of material parameters (properties) in porous materials

I am using poroelasticity module. There are porous material and fluid in the model. When I assign the properties of porous material or fluid, I need to choose which domain I should assign. The fluid and porous materials are at the same domain, because fluid exists in the void of porous materials. So after I assign the properties of the porous materials, for example, density, to a certain domain, I need to assign the properties of fluid to the same domain. But the question is that after I assign the properties of fluid to this domain, the assignment of properties of porous materials will be overridden by the properties of fluid.

So my question is that how do I assign the material parameters of fluid which exists in the void of porous materials to the domain? Thanks