[bes]

var x = [4, -3, 8, 2, 7];
var y = [-5, 12, 9, 0, -1, 6, 6];
var mas = x.slice(); 
var len = y.length;
var num;

for (var i = 0; i < len; i++) {
  num = Math.floor(mas.length * Math.random());
  mas.splice(num, 0, y[i]);
}
alert('x = [' + x + ']\ny = [' + y + ']\nmas = [' + mas + ']');