Fill One TextBox from two

function checkthis2() { if (document.getElementById("TextBox35").value != "") { document.getElementById("TextBox36").innerHTML = ""; $("#TextBox36").css("visibility", "hidden"); } else if (document.getElementById("TextBox36").value != "") { document.getElementById("TextBox35").innerHTML = ""; $("#TextBox35").css("visibility", "hidden"); } else if (document.getElementById("TextBox35").value == "") { $("#TextBox36").css("visibility", "visible"); } else if (document.getElementById("TextBox36").value == "") { $("#TextBox35").css("visibility", "visible"); } if (document.getElementById("TextBox36").value == "" && document.getElementById("TextBox35").value == "") { $("#TextBox36").css("visibility", "visible"); $("#TextBox35").css("visibility", "visible"); } This is first Textbox This is second Textbox