When multiple databases contain the same material, STAR-CCM+ uses the property values from the first database loaded with that material. STAR-CCM+ ignores property values for that material in subsequently loaded databases, whether or not any of those values may be different.
If any materials vary from one database to another, all materials from all the databases you load are brought into the simulation. Suppose one database contains a material called Alloy1, and the other database contains a material called Alloy2. After you load both databases with these materials, both materials will be included in the simulation.
Although the databases appear to be merged in the simulation's object tree, neither the default props.dbs file nor the one(s) that you load are modified.