aradığın bu sanırım.

Default.aspx









Panel1


Panel2











Default.aspx.cs

protected void Button1_Click(object sender, EventArgs e)
{
Panel1.Visible = true;
Panel2.Visible = false;
}
protected void Button2_Click(object sender, EventArgs e)
{
Panel1.Visible = false;
Panel2.Visible = true;
}