попробуйте
$arr1=array(11,34,30,35);
$arr2=array(12,666,23);
$odd= array_combine(range(0,2*count($arr1)-1,2), $arr1);
$even = array_combine(range(1,2*count($arr2)-1,2), $arr2);
$output=$odd+$even;
ksort($output);
echo "<pre>";
print_r($output);
возвращает
Array
(
[0] => 11
[1] => 12
[2] => 34
[3] => 666
[4] => 30
[5] => 23
[6] => 35
)