I dual boot, and that would be very nice.
I use Windows 7 and Crunchbang Linux by the way.
I googled this question and all I got was how to sync the library between two devices - and that's not what I want to do.
Simply have the media in a partition which can be accessed by both OS-es. The easiest way would be to keep the media on the Windows partition, or a separate NTFS partition that is dedicated to media. You can mount that partition in Linux using ntfs-3g driver.
XBMC metadata collection would be performed separately on both OS installations. You would have to configure them separately as well. Once that is done, you should be able to enjoy a hassle-free media experience.
You will probably hit problems if you try to share the XBMC metadata database between two different OS installations.
If you wanted to attempt it, one option would be to set up a MySQL database that is shared between Windows and Linux:
Then setup both the windows and linux installation to use this database:
The file paths used by Linux will more than likely be different to those used within Windows, so you would have a challenge trying to configure both systems to share a common set of filepaths. Any mismatches and you would end up with duplicate entries within the database, entries being removed, media not being found or accessible etc.
A simpler approach would be to store your media in a partition that both OS's can see, and just have 2 separate sets of metadata, one for each OS.
Alternatively for a more flexible solution, get yourself a network storage NAS box or a cheap PC you can use as a media server, install the database and all of your media onto that and then read it from any XBMC device on your home network with one single metadata source.