In the Hitchhiker's Guide to the Galaxy books, there are only a few cases where it is specified which head Zaphod Beeblebrox talks out of (these are typically when one head is drunk or eating, as far as I can tell). In the movie version, on the other hand, it seems that Zaphod's "normal" head talks most of the time, while his second head, apparently attached by his neck, talks when Zaphod is acting less presidential.
Is there any material that explains which head Zaphod normally talks with in the books and/or radio series (the original head or the added one)? I noticed that his heads appear to be side-by-side; one is referred to as the "left head" and one as the "right head".