The only way to do this is to loop through one array and a nested loop through the second array and concatenate each value individually. There is no single php function that will automatically do this work.
$class = get_field('AAA');
$tijd = get_field('BBB');
$value = array();
foreach ($class as $a) {
foreach ($tijd as $b) {
$value[] = $a.$b;
}
}
$value = implode(', ', $value);