1) Two groups of "Prev by" , one for subject and date OR
You have encountered a limitation in MHonArc. You can only have
prev/next links for the 2 default indexes. One being thread, the other
being one of the main index types: date, subject, author, or message
Therefore, you cannot have prev/next links for subject and date
at the same time.
2) Have the "Prev by" change from subject to date depending on the index
it is choose from.
If you are refering to "on-the-fly" changing, that cannot be done
with the way mhonarc is now. That's a CGI-type function.
The best thing I can think of now is to use the thread index as pseudo
a subject index. Use the new TSUBSORT resource and set the TLEVELS to
1. Have the main index sort by date, and change resource that say
"thread" to subject. With this, you do not need OTHERINDEXES, and you
get prev/next for both indexes.