There are reports of twins bootstrapping off each other, from the principle of noise->action->repeatnoise, called idioglossia. Seems is not that great actually as language. This NYT blog post suggests the words are more babble than language, which matches how my daughter spoke to the cat: English intonation and facial expressions, meowy babble as words. The Wikipedia article on cryptophasia says "While sources claim that twins and children from multiple births develop this ability perhaps because of more interpersonal communication between themselves than with the parents, there is inadequate scientific proof to verify these claims."