How to use multiple Steam accounts without re-entering passwords?

I'd like to switch between two Steam accounts (from the same windows user account) without having to re-enter their passwords. So basically I am trying to write a batchfile which takes care of the relevant files and/or registry settings, but for that I need to know

Which files and registry settings change when logging in as another user?

Obvious bonus points for providing a batch file for that task.

The Steam executable (Steam.exe on Windows) accepts a number of command line parameters; you could try taking advantage of those.

Specifically, this one, which starts Steam and immediately attempts to log in with the given credentials:

-login %u %p

This logs into Steam with the specified Username and Password combination. Replace %u with the username, and %p with the password you want to login with (Steam must be off for this to work).

And, since this doesn't work while Steam is running:

-shutdown

This shuts down (exits) Steam.

You can run TWO Steams with Sandboxie.
(Yeah, I know this is not what you want.
Still, this might worth a try if anyone is interested in such a solution.)