Объединение

using System;
using System.Collections.Generic;
using System.Linq;

namespace Unification
{
    class Program
    {
        static void Main(string[] args)
        {
            string lastNamePrefix = "Б";

            List<Soldier> platoon1 = new List<Soldier>()
            {
                new Soldier("Иван", "Ботвинник"),
                new Soldier("Сергей", "Васильев"),
                new Soldier("Иннокентий", "Смактуновский"),
                new Soldier("Григорий", "Ростовский"),
                new Soldier("Армен", "Батура"),
                new Soldier("Сергей", "Фрай"),
            };

            List<Soldier> platoon2 = new List<Soldier>()
            {
                new Soldier("Александр", "Тастатура"),
                new Soldier("Сергей", "Божьев"),
                new Soldier("Влас", "Кузнецов"),
                new Soldier("Артемий", "Гусев"),
                new Soldier("Анатолий", "Бацман"),
                new Soldier("Сергей", "Касперский"),
            };

            var relocatingSoldiers = platoon1.Where(soldier => soldier.LastName.StartsWith(lastNamePrefix));
            platoon2 = platoon2.Union(relocatingSoldiers).ToList();
            platoon1 = platoon1.Except(relocatingSoldiers).ToList();
            Console.WriteLine("  Первый отряд:");

            foreach (var soldier in platoon1)
                soldier.ShowInfo();

            Console.WriteLine("\n  Второй отряд:");

            foreach (var soldier in platoon2)
                soldier.ShowInfo();
        }
    }

    class Soldier
    {
        public Soldier(string name, string lastName)
        {
            Name = name;
            LastName = lastName;
        }

        public string Name { get; private set; }
        public string LastName { get; private set; }

        public void ShowInfo()
        {
            Console.WriteLine($"{Name} {LastName}");
        }
    }
}