Putting down the summary of our discussion so far.
Wakanda doesn't seem to support a concept of combined unique key.
Alternatively, uniqueness of an entity can be checked via OnValidate() entity event function by checking existence of a record with specific attribute values. But not sure what happens if multiple threads execute OnValidate() for an entity having same attribute values at the same time? There is a chance of having duplicate entities gets saved, isn't it?
Please share if you have better thoughts/ideas.