They aren't compatible, however, MC12 will automatically convert it as it restores an MC11 backup to MC12.
You can always restore an earlier backup to a newer version of MC, but you can't go backwards, i.e.: you can't restore an MC12 backup to MC11.
Your MC11 backups will not be affected as MC12 will put the newly converted library in it's own MC12 Library Backups folder.