@Database(entities = {/*... the first set of entities ...*/}, version = 1)
public abstract class AppDatabase1 extends RoomDatabase {
    // the first set of DAOs
}

@Database(entities = {/*... the second set of entities ...*/}, version = 1)
public abstract class AppDatabase2 extends RoomDatabase {
    // the second set of DAOs
}

AppDatabase db1 = Room.databaseBuilder(getApplicationContext(), AppDatabase1.class, "database1.db").build();

AppDatabase db2 = Room.databaseBuilder(getApplicationContext(), AppDatabase2.class, "database2.db").build();

@Entity
public class User {
    @PrimaryKey
    @NonNull
    public String uid;

    @ColumnInfo(name = "first_name")
    public String firstName;

    @ColumnInfo(name = "last_name")
    public String lastName;
}

@Dao
public interface UserDao {
    @Query("SELECT * FROM user")
    List<User> getAll();

    @Query("SELECT * FROM user WHERE uid IN (:userIds)")
    List<User> loadAllByIds(int[] userIds);

    @Query("SELECT * FROM user WHERE first_name LIKE :first AND " +
           "last_name LIKE :last LIMIT 1")
    User findByName(String first, String last);

    @Insert
    void insertAll(User... users);

    @Delete
    void delete(User user);
}

@Database(entities = {User.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

public class DatabaseClient {

  private Context mCtx;
  private AppDatabase appDatabase;
  private static String databaseName;
  private static DatabaseClient mInstance;


  private DatabaseClient(Context mCtx, String dbName) {
    this.mCtx = mCtx;

    if(databaseName == null || !databaseName.equalsIgnoreCase(dbName)) {
      databaseName = dbName;
    }
    appDatabase = Room.databaseBuilder(mCtx, AppDatabase.class, databaseName).build();
  }

  public String getDatabaseName() {
    return databaseName;
  }

  public static synchronized DatabaseClient getInstance(Context mCtx, String dbName) {
    if (mInstance == null || databaseName == null || !databaseName.equalsIgnoreCase(dbName)) {
      mInstance = new DatabaseClient(mCtx, dbName);
    }
    return mInstance;
  }

  public AppDatabase getAppDatabase() {
    return appDatabase;
  }
}

List<User> users = DatabaseClient.getInstance(getApplicationContext(), myDb).getAppDatabase().userDao().getAll()