Depends which one you want to open first, the one u want to open first open it on the click of the button and the other one on the page load fuction of the opening one.
string strJavascript;
strJavascript = "<script language='javascript'>";
strJavascript = strJavascript + " window.showModalDialog('AddOccupantEdit.aspx?id=-100','text','status=0,toolbar=0,width=750,height=320'); </script>";
Page.ClientScript.RegisterStartupScript(this.GetType(), "__close", String.Format(strJavascript));