Alternate keys in entity framework core

I want to have a 2 alternate keys with this scenario :

public class A
{
  public int Id {get;set}
}

public class B:A
{
  public int OtherPropertyKey {get;set}
  public string Name {get; set}
}

DbContext and ExtensorMethod :

    public class CContext : DbContext
    {

        public DbSet<B> Bs{ get; set; }

        public CContext (DbContextOptions options) : base(options)
        {
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.ApplyConfigurationsFromAssembly();

            base.OnModelCreating(modelBuilder);
        }
    }



    public static class EfExtensions
    {
        public static void ApplyConfigurationsFromAssembly(this ModelBuilder modelBuilder)
        {
            var allClasses = AllClasses.FromLoadedAssemblies().ToList();

            var conftypes = allClasses.Where(t => !t.IsInterface && !t.IsAbstract && t.GetInterfaces()
                                                      .Where(i => i.IsGenericType)
                                                      .Select(i => i.GetGenericTypeDefinition())
                                                      .Any(i => i == typeof(IEntityTypeConfiguration<>)))
                .Select(t => new
                {
                    ConfigType = t,
                    //EntityType =
                    //t.GetInterfaceGenericArguments(typeof(IEntityTypeConfiguration<>)).Single()
                }).ToArray();

            foreach (var conf in conftypes)
            {
                dynamic confInstance = Activator.CreateInstance(conf.ConfigType);
                modelBuilder.ApplyConfiguration(confInstance);
            }
        }
    }

Implementation of IEntityTypeConfiguration:

    public class BConfiguration : IEntityTypeConfiguration<B>
    {
        public void Configure(EntityTypeBuilder<B> builder)
        {
            builder.HasKey(b => new {b.Id, b.OtherPropertyKey});
            builder.Property(b => b.Id).ValueGeneratedOnAdd();
            builder.Property(b => b.OtherPropertyKey).ValueGeneratedOnAdd();
        }
    }

When I do a migration with command dotnet I get this:

migrationBuilder.CreateTable(
                name: "B",
                columns: table => new
                {
                    Id = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    Name = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {table.PrimaryKey("PK_Bs", x => x.Id);  /// I want two PK like : table.PrimaryKey("PK_Bs", x => x.Id);
                                                           ///                      table.PrimaryKey("PK_Bs", x => x.OtherPropertyKey);
                });

Whay do I got that ??? What do I doing wrong ???