$data1 = file_get_contents("/tmp/msgpack1");
$data2 = file_get_contents("/tmp/msgpack2");

$unpacker = new \MessagePackUnpacker(false);
$unpacker->feed($data1);
if($unpacker->execute()) //return TRUE
{
    $unpacked = $unpacker->data();
    print_r($unpacked); //here we have partial data, sometime we have Exception, sometime other errors
}

$unpacker->reset();

$unpacker = new \MessagePackUnpacker(false);
$unpacker->feed($data1 . $data2);
if($unpacker->execute()) //return TRUE as expected
{
    $unpacked = $unpacker->data();
    print_r($unpacked); //all is OK
}

$unpacker->reset();

#: php -v
PHP 7.0.13 (cli) (built: Nov 11 2016 17:35:21) ( NTS )