public partial class Form1 : Form
{
    HashSet<Keys> pressedKeys = new HashSet<Keys>();

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        pressedKeys.Add(e.KeyCode);
        HandleChange();
    }

    private void Form1_KeyUp(object sender, KeyEventArgs e)
    {
        pressedKeys.Remove(e.KeyCode);
        HandleChange();
    }

    void HandleChange()
    {
        label1.Text = pressedKeys.Contains(Keys.Space) && pressedKeys.Contains(Keys.D)
            ? "Stisknuto"
            : string.Empty;
    }
}