If you're one of the ten people who haven't seen the twin-baby-conversation video, here you go:
(Part 1 is here.)
As Geoff Pullum noted in email,
It isn't as funny as the Ranting Toddler, but it sure does suggest that the idea of conversation, complete with intonation and hand gestures, emerges way before even a single word is learned.
At 17 months of age, these guys are old enough that they also probably have some words, but in this interaction they're obviously working on something besides vocabulary development.