currently Dokuwiki can use only one CSS file per system yet mixing RTL and LTR pages requires the use of two CSS which are called uppon demand.
MoinMoin has a nice implementation of such feature (see
http://moinmoin.wikiwikiweb.de/HebrewSupport#head-03f18b8c6282a8f053653bd8a3c0f8f74a7e2e56 )