在 github 上查看 ASP.NET Core 源代码后，可以使用此扩展方法添加第二个身份:

After looking through the ASP.NET Core source code on github, a second identity could be added using this extension method:

using Microsoft.AspNetCore.Identity;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;
using System;
using System.Collections.Generic;
using System.Text;

namespace Whatever
{
    public static class IdentityExtensions
    {
        public static IdentityBuilder AddSecondIdentity<TUser, TRole>(
            this IServiceCollection services)
            where TUser : class
            where TRole : class
        {
            services.TryAddScoped<IUserValidator<TUser>, UserValidator<TUser>>();
            services.TryAddScoped<IPasswordValidator<TUser>, PasswordValidator<TUser>>();
            services.TryAddScoped<IPasswordHasher<TUser>, PasswordHasher<TUser>>();
            services.TryAddScoped<IRoleValidator<TRole>, RoleValidator<TRole>>();
            services.TryAddScoped<ISecurityStampValidator, SecurityStampValidator<TUser>>();
            services.TryAddScoped<IUserClaimsPrincipalFactory<TUser>, UserClaimsPrincipalFactory<TUser, TRole>>();
            services.TryAddScoped<UserManager<TUser>, AspNetUserManager<TUser>>();
            services.TryAddScoped<SignInManager<TUser>, SignInManager<TUser>>();
            services.TryAddScoped<RoleManager<TRole>, AspNetRoleManager<TRole>>();

            return new IdentityBuilder(typeof(TUser), typeof(TRole), services);
        }
    }
}