The chapter is concerned with simultaneous bilinguals (2L1s) assuming that they are a subtype of the heritage speaker (HS). The chapter summarizes major findings with regard to their morpho-syntactic and phonological characteristics, starting with the two competing views on bilingual language development that postulated fused language systems as opposed to autonomous development several decades ago. In the 1990s, when the idea of separate systems had been widely accepted, research has concentrated on the question under which conditions the two languages of bilingual children show unidirectional or bidirectional influence—a question which is also relevant in the current literature on adult HSs. The present chapter relates findings from developing heritage bilinguals and adult HSs, discussing divergent acquisition outcomes that diverge from those of monolingual with regard to cross-linguistic influence, age of onset, input and language dominance, as well as distance to the homeland.
