First of all what is couple number? I am guessing you are talking about even number.
Anyway, If you write on innerHTML on a loop then you will get only the final result.
function check(){
var a = Number(document.getElementById("tiv1").value);
var b = Number(document.getElementById("tiv2").value);
var count = 0;
var evenNUmbers = new Array();
for(var i = a; i <= b; i++){
if(i % 2 == 0){
count++;
evenNUmbers.push(i);
}
}
document.getElementById("result").innerHTML = "There are " + count + " couple numbers " + " ( " + evenNUmbers.join() + ")";
}