Formalist and early Structuralist thinkers failed to articulate a sociological, "living language" in their literary analyses. Bakhtin recognized this and referred to this mistake as a "monologic" view of language. This monologic view considers language to be an objective system from an author can partake and control his/her own discourse (control what he writes and how it is read). In this view, language is a set system. Each reader/hearer should read/hear the same set of words in a similar way.
Especially in regards to novels, Bakhtin thought that this view is not sufficient to understand the dynamics of the novel. Similar to intertextuality, Bakhtin uses the term "heteroglossia" to describe how there are many voices in a novel. An author has one voice but is able to utilize different voices (based on dialect, social class, groups, cultures, generations, etc.) in voicing characters and their perspectives in the novel. Bakhtin says this is the characteristic that distinguishes the novel from other forms of literature (namely, poetry). In the novel, different voices change each other and they see things in different ways. There is a conflict of ideas in the novel; not so much in poetry.
This comes from Bakhtin's belief that language itself is dialogic (as opposed to monologic). Likewise, the novel is dalogic. If language and the novel both illustrate heteroglossia or dialogism, then what we are talking about is the notion of many voices talking to each other (in the novel and in the author's mind) and influencing one another. This is a similar, but a more nuanced way of describing intertextuality which was more precisely about the interconnected relationship between texts. In short, those who speak of heteroglossia (Bakhtin) and those who articulate intertextuality (Kristeva) are talking about a similar notion, just employed in different ways. Bakhtin's idea of heteroglossia comes from analyses of how the novel shows the many subjective and conflicting voices of real life, each influencing the other. Intertextuality shows how all texts influence each other. To come closer to Bakhtin's way of thinking, with intertextuality, it's as if all texts are in a constant dialogue with one another. In both cases, the continual dialogue yields new meanings and interpretations (within the novel itself and as it relates to other texts--intertextuality--over time).