MOTM

:> Is there a clever way to handle it *duophonically* at least?
:> Like a bow working on two strings at the same time? (I'm
:> not a violin player, so I can only guess, but I think they
:> do this, don't they?)


Violins, and other bowed instruments are fully capable of double and triple
stops, though, due to tuning restrictions, only certain combinations of
notes are possible. Forget tight tone clusters, for example..

There's a reference here:

http://gigue.peabody.jhu.edu/~mathews/Orchestration/String%20Notes/multistop
s.html

or grab a book:

I've always like "Principles of Orchestration" by Rimsky-Korsakov, but some
folks don't as it's pretty specific to his own music....