Seems like you can use the same Array.prototype.slice.call that makes the args array-like object become an array. (See here)
var inputs = document.getElementsByTagName('input');
var selects = document.getElementsByTagName('select');
inputs = Array.prototype.slice.call(inputs);
selects = Array.prototype.slice.call(selects);
var res = inputs.concat(selects);
alert(res.length);