var arr1 = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];

var arr2 = [1, 2, 3, 4, 5, 6, 7, 8];

var arr3 = [];

​

arr1.forEach(function(value, index) {

  arr3.push(value);

  if (index % 2 == 1) {

    arr3.push(arr2.shift()); // Изменяет arr2!

  }

});

​

console.log(arr3);