This does not work. The problem is f.e. rule 0 has only one plural case, therefor it will not be able to add an "and" into the list.
https://wiki.phpbb.com/Plural_Rules#Rule_.230

You will need to just use 2 different strings. See `ONLINE_USERS_TOTAL` and `ONLINE_USERS_TOTAL_GUESTS` f.e. https://github.com/phpbb/phpbb/blob/develop/phpBB/language/en/common.php#L505