You wouldn't be able to use the same install, but theoretically (I haven't tried this), you could do something like:
1. Install windows using Boot Camp on a FAT32 partition (just to be sure, not sure if it needs to be FAT32).
2. Install windows on Parallels.
3. Map the boot camped partition as a network drive in the Parallels install.
4. This is where it gets hazy ... point all the directories, e.g. "Program Files" and "My Documents" to the ones on the mapped boot camp partition. I say "hazy" because, although installed executables would be available, they wouldn't be registered in the Windows Registry in Parallels, and I'm not sure how well they would run, if at all. And you still need two installs. Documents and data files could be shared this way, though.