how to make 2 forms in C# be displayed in the same form

Assume:

1. Form A is your main form in a Win Form Application.
2. Form A will create Form B.
3. There's a Button on Form A that if clicked will hide Form A and show Form B
4. There's a Button on Form B that if clicked will hide Form B and show Form A.
5. The Form that is hidden should not show in the TaskBar.

Form A:

public partial class FormA : Form
{
    public FormA()
    {
        InitializeComponent();
    }

    FormB B = new FormB();
 
    // Form A Load EventHandler   
    private void Form1_Load(object sender, EventArgs e)
    {
        this.ShowInTaskbar = true;

        // subscribe to the Click Event of Form B's button
        B.FormBBtn.Click += btnShowA_Click;
    }

    // the Click EventHandler for the Button on FormB
    private void btnShowB_Click(object sender, EventArgs e)
    {
        this.ShowInTaskbar = false;
        this.Hide();

        B.ShowInTaskbar = true;
        B.Show();
    }

    // the Click EventHandler for the Button on FormA
    private void btnShowA_Click(object sender, EventArgs e)
    {
        B.ShowInTaskbar = false;
        B.Hide();

        this.ShowInTaskbar = true;
        this.Show();
    }
}

Form B:

public partial class FormB : Form
{
    public FormB()
    {
        InitializeComponent();

        // set the reference to Form B's Button
        FormBBtn = btnShowA;
    }

    // expose Form B's Button via a Public Property
    public Button FormBBtn { get; private set; }
}

Note that we must set the reference to Form B's Button in Form B's Constructor because the instance of Form A needs access to it before the Load Event for Form B will be triggered in order to subscribe to Form B' Button Click Event.