EDIT: Looks like this is coming, according to Google employee Colin Gibbs, on this Google Mobile Support forum thread:
Thanks for being patient while we work
on this feature. When we launched
Android 3.0 (Honeycomb), we added
support to Google Talk for multiple
accounts. We're currently working on
bringing the feature to phones as
So it looks like it should work on a Honeycomb tablet, and they may be backporting it to Gingerbread phones (video chat over GTalk has just been enabled for 2.3.4, so they're definitely porting Honeycomb Talk's features back to phones).
Not a great solution, doesn't do all the alerting and notifications in a way that's integrated with the Android OS, but the only way to do this currently seems to be to use the built-in GTalk client for one account, and access the other via the web page.
For a standard GTalk account use this URL to sign into the mobile webpage:
For a Google Apps Gtalk account use this URL to sign into the mobile webpage:
Or use a second IM client for the second GTalk account, as suggested by Matthew Read.