Fixed

using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Numerics;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApp9
{
    class Program
    {
        static void Main(string[] args)
        {

            string input = Console.ReadLine();

        string pattern = @"^[a-zA-z]\w{2,24}$";
        string[] usernames = input
            .Split(" /\\()".ToCharArray(), StringSplitOptions.RemoveEmptyEntries)
            .ToArray();
        List<string> users = new List<string>();

        foreach (string user in usernames)
        {
            if (Regex.IsMatch(user, pattern))
            {
                users.Add(user);
            }
        }

            int maxCount = 0;
            string temp1 = string.Empty;
            string temp2 = string.Empty;

            for (int i = 0; i < users.Count - 1; i++)
            {
                if (users[i].Length + users[i + 1].Length > maxCount)
                {
                    maxCount = users[i].Length + users[i + 1].Length;
                    temp1 = users[i];
                    temp2 = users[i + 1];
                }
            }

            Console.WriteLine($"{temp1}\n{temp2}");
        }
    }
}