I think the simplest solution would be to download the official portable version from portableapps dot com. Set one up with your accounts and set up the other with her accounts. The only downside is that clicking a 'mailto' would go to the default installation, so if you two decide who is more likely to do that, the portable version could go to the one who does not need 'mailto'. The bright side is that gives each of you full control of addressbooks, addons, and any other tweaks, such as preferred font.

Set up separate profiles, one with just the accounts used while traveling. Even if you uncheck all the settings for getting mail in Server Settings, an IMAP account might still 'push' messages to TB if the IMAP server supports the IDLE command.

Create and manage profiles from Help/More Troubleshooting, about:profiles. Instead of launching TB from the normal shortcut, open Profile Manager by opening a Run window (Winkey+R) and typing thunderbird.exe -p Select the appropriate profile.

sfhowes' solution is the superior one. Just right-click on desktop and add -p to the end of the target statement and you will be prompted each time you start TB on which profile to use. I suggest also using profile manager the first time to name each profile uniquely, e.g., HIS and HERS.

Thanks to both david and sfhowes for your suggestions. I decided to just have the two profiles (his & her) and I can flip back and forth between them when travelling or when I need to see something in her inbox. I just love user forums, you generally get far better answers than the official company support personnel give. Thanks once again.