Warning -- this is just my reading of the manual. I haven't done it.
1. Make sure that you control all resources except RCFILE from an .rc file.
2. Copy your .rc file under a new name.
3. Edit the copy to invoke SUBJECTSORT and NOTHREAD
4. Add a clause to the copy giving a name for the index file
which does not conflict with the name of the current main index.
4. Edit the original .rc file to name the copy in OTHERINDEXES.
Now clean things up. Tune the formating of the subject index as
desired, by modifying the resource utilization in the secondary
or "copy" .rc file. Add/change references to the subject index
on the pages for the message and for the existing indices as