how to add two inline buttons to a telegram bot by c#?

You can do it by pass IEnumerable<InlineKeyboardButton> to InlineKeyboardMarkup Like ineasted of pass InlineKeyboardButton like this this:

private void Main(string[] args)
{
    StringBuilder sb = new StringBuilder();
    
    // Buttons
    InlineKeyboardButton urlButton = new InlineKeyboardButton();
    InlineKeyboardButton urlButton2 = new InlineKeyboardButton();

    urlButton.Text = "Go URL1";
    urlButton.Url = "https://www.google.com/";

    urlButton2.Text = "Go URL2";
    urlButton2.Url = "https://www.bing.com/"
    
    
    InlineKeybordButton[] buttons = new InlineKeybordButton[] { urlButton, urlButton2 };
    
    // Keyboard markup
    InlineKeyboardMarkup inline = new InlineKeyboardMarkup(buttons);
    
    // Send message!
    bot.SendTextMessageAsync(chatId, sb.ToString(), ParseMode.Html, true, false, 0, inline);
}

And you can control How many buttons in every row By Passing IEnumerable<IEnumerable<InlineKeyboardButton>> to InlineKeyboardMarkup constructor And Every IEnumerable it will be a single row like this:

    // Defining buttons
    InlineKeyboardButton urlButton = new InlineKeyboardButton();
    InlineKeyboardButton urlButton2 = new InlineKeyboardButton();
    InlineKeyboardButton urlButton3 = new InlineKeyboardButton();
    InlineKeyboardButton urlButton4 = new InlineKeyboardButton();

    urlButton.Text = "Go URL1";
    urlButton.Url = "https://www.google.com/";

    urlButton2.Text = "Go URL2";
    urlButton2.Url = "https://www.bing.com/";

    urlButton3.Text = "Go URL3";
    urlButton3.Url = "https://www.duckduckgo.com/";

    urlButton4.Text = "Go URL4";
    urlButton4.Url = "https://stackoverflow.com/";
     
    // Rows, every row is InlineKeyboardButton[], You can put multiple buttons!
    InlineKeyboardButton[] row1 = new InlineKeyboardButton[] { urlButton };
    InlineKeyboardButton[] row2 = new InlineKeyboardButton[] { urlButton2, urlButton3 };
    InlineKeyboardButton[] row3 = new InlineKeyboardButton[] { urlButton4 };


    // Buttons by rows
    InlineKeyboardButton[][] buttons = new InlineKeyboardButton[][] { row1, row2, row3 };
    
    // Keyboard
    InlineKeyboardMarkup keyboard = new InlineKeyboardMarkup(buttons);

    // Send Message
    await bot.SendTextMessageAsync(chat, "Message", replyMarkup: keyboard);

And this Is the result in second time:

More simpler compact syntax:

var inlineKeyboard = new InlineKeyboardMarkup(new[]
            {
                InlineKeyboardButton.WithUrl("Go url 1", "https://www.google.com/"),
                InlineKeyboardButton.WithUrl("Go url 2", "https://www.bing.com/")
            });

Message message = await bot.SendTextMessageAsync(
                    chatId,
                    text,
                    replyMarkup: inlineKeyboard,
                    parseMode: ParseMode.Markdown);