Не прокатило.
var Arr1 = ["1", "2", "3", ["1","2","3"]];
var Arr2 = [[1,2],[1,2]];
Arr2.unshift(Arr1.slice());
alert(Arr2[0]);
Arr1[3][1] = "BEDA";
alert(Arr2[0][3][1]); //"BEDA"
var Arr1 = ["1", "2", "3", ["1","2","3"]];
var Arr2 = [[1,2],[1,2]];
Arr2.unshift([].concat(Arr1));
alert(Arr2[0]);
Arr1[3][1] = "BEDA";
alert(Arr2[0][3][1]); //"BEDA"