TB doesn't work with identities like OE. It's more or less based on the idea that another user would login to Windows as another user. Not that all users use the same login to Windows.

However, you can set TB to use a different path in the registry and set the different registry trees to use different mail directories.
To do that you should use the /Reg command line parameter. As it was developed for beta testing, it's possible that the help file doesn't mention it.

Basically it works like this, mummy starts TB with the command 'thebat /Reg:mum' and daddy starts TB with the command 'thebat /Reg:dad' and they'll use different settings and will be able to use different mail directories (that's important, otherwise ytou can't use different accounts, address books, preferred settings, etc.) Of course you can set the command line via short links on the desktop

This command line parameter has been discussed previously on the forum, so you might search for more info.