Hi Paul,
I went through a very similar situation at the end of last year with a client of mine. That client's business broke apart into two other companies. It was scary because that old company's website had a lot of authority, ranked for many, many terms (long tail and head terms).
Adam's recommendation of getting links to the new domains is critical. In the case of my client, one of the new websites did have a few links (a dozen or so) while the other website had no links. The website without links did not perform as well. I would recommend you start that process now, even if those links point to a "coming soon" type home page on those new domains.
For the home page, we created a splash page that offered visitors a choice of going to one company or the other. We did put deeper links into both sites on this page, though that was more a usability consideration. I wouldn't redirect the home page to either website, unless there is a clear usability case to be made for doing so. That way the splash page acts a communication to users and Google about the new direction of the website.
Also, so that you know...Google still has that page indexed but after the first 2-3 months, that page started to lose rankings and traffic from search. I'm not sure if it lost rankings because of it being a splash page or because we started changing links (I suspect the links though the change in the content of the home page certainly played a role). Either way, that was the desired intention; you don't want the splash page to rank.
All the other pages on the site were 301 redirected to the appropriate pages of the new websites. We had no troubles redirecting to multiple domains. Adam is right though that this may be seen as spammy so be careful with this. We started out in small batches of pages as a test before going full on into the redirects. If you can, I'd start with that. Measure, test, repeat with another small batch, etc.
Finally, I will tell you that we did see a drop in rankings/traffic to the two new websites (that is, the total Google and Bing activity for the two sites was less than where we had been with the old website). We are now at the seven month mark and starting to regain steam. Not quite where it was this time last year, but I am seeing gains. SO, make sure that you brace for impact and plan for a slowdown when you do this.
I hope I was able to help you out. Thanks,
Matthew