An RSS Bandit user has created the RSS Bandit Flagged Item Merge Utility. The description of its usage states

If you use RSSBandit as your feedreader, you've probably used the Flag Item feature to store interesting articles for future access. However, if you use RSSBandit on multiple computers, your flagged item lists aren't synchronized. While you could upload your data to a central location, I find it more convenient to use a USB Memory Stick to keep these flagged item lists. I wrote this utility to synchronize from various lists to your main list. Get it here.

Usage: Select your main flagitems.xml file as the RSS Bandit Flag File and the file you want to merge into it as the Import File and hit Start. The application will display the items that it imported successfully or failed.

Although it is quite cool to see people writing tools to work with RSS Bandit, the fact is that you can already get this functionality out of RSS Bandit without resorting to tools. One of the options in the Remote Storage tab of the Tools->Options menu is 'File Share'. Although this option states that it only works on network shares it also works on local drives as well. So this means you can select an external drive such as "H:\" or whatever drive letter the USB keychain maps to and then synchronize with that.

That way all you need to synchronize RSS Bandit between two machines would be your USB keychain. I guess this means we should probably update the text of that dialog to explain that it's actually any drive not just network shares.