Edited and fixed (thanks to Shredder)
If you mean you want to open a new tab, try the below:
protected void Page_Load(object sender, EventArgs e)
{
this.Form.Target = "_blank";
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Redirect("Otherpage.aspx");
}
This will keep the original page to stay open and cause the redirects on the current page to affect the new tab only.
-J