I know this post is sooo old, but for those who happen upon it while looking for the same thing as me:
There are likely easier ways to achieve this in a shell... if you know, share.
The following will likely work on other versions of Windows and possibly other operating systems.
On Windows XP SP3, with most recent Tor Browser Bundle as of 3/28/12:
Extract the Tor Browser Bundle.
Run "Start Tor Browser.exe," or its equivalent on your other operating system.
Right-click the Vidalia (onion) icon in your system tray, after waiting a few moments.
Select "Settings."
Select "Advanced."
Check the box for "Configure ControlPort automatically," or find its equivalent in the program's GUI settings for your operating system.
Right-click the Vidalia icon and select "Exit."
Copy the extracted "Tor Browser" directory for each desired instance of a separate IP to "Tor Browser 2", "Tor Browser 3", etc.
Start them successively, waiting for each to complete before attempting the next.
We now has multiple IP addresses, esp. useful for simultaneous downloads using free accounts from services such as Filesonic.
If this is not the solution you desire, please be more specific in your post (OS, ultimate goal, steps taken, etc.). Multiple instances of TOR itself (sans browser, secure settings, and management applications like Vidalia) are actually much easier than the above. Message me if you have any further questions.