$test = $collect
  ->groupBy('product')->map(function ($item) {
    return $item[0];
  })->toArray();

$test_2 = $collect_2
  ->groupBy('product')->map(function ($item) {
    return $item[0];
  })->toArray();

foreach ($test as $key => $item) {
  $test_3[$key] = array_merge($test[$key] ?? [], $test_2[$key] ?? []);
}