The problem with this system is that most operating systems and file managers have no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.
A variation of this are copies of the source file which contain largely the same information, but in a different format or from a previous version: Rather than storing data separately, it can be stored as part of the main file.
These can then be manipulated with usual file system tools: because the support is built into the operating system, these resource forks will not show up as separate files, and all applications inherit support for resource forks.
However, this causes problems with exchanging over ISO 9660 format CD-ROM, FAT format MS-DOS disks, and over internet email, and requires the use of sidecar files to store this information.
Microsoft NTFS supports Alternate Data Streams which are similar.