class Form2 : Form{

    //中略

    public event EventHandler AddButtonClick;
    private void button1_Click(object sender, System.EventArgs e)
    {
        if(AddButtonClick!=null) AddButtonClick(this, EventArgs.Empty);
    }
}

public class Form1 : Form{

    //中略

    private void Form2を表示(){
        Form2 f = new Form2();
        f.AddButtonClick += new EventHandler(form2_AddButtonClick);
        f.Show();
    }

    private void form2_AddButtonClick(object sender, EventArgs e){
        this.tabControl1.TabPages.Add(new TabPage("たｂ"));
    }
}