Square-tailed and fork-tailed are two sub-species of the same bird, which, as far as I know, haven't been split yet. Square-tailed drongo-cuckoo is resident in South East Asia, while Fork-tailed is a winter migrant there (and is the sub-species that occurs in Hong Kong). The square-tailed drongo-cuckoo is supposed to have a slightly shallower forked tail. From my observations of resident birds in Malaysia and Singapore, they both actually have fork-tailed (and are not always easy to separate in the field).
