If you refer to the RFA_ConfigGuide.pdf for the fileLoggerFilename parameter, it mentions the ability to include things like Process ID in the filename.
e.g. \Logger\AppLogger\fileLoggerFilename="rfa{p}.log"
will generate a log filename named something like rfa1234.log
I have just tested briefly here with RFAC++7.4 on Windows and ran two concurrent instances of the same app and two seperate rfaxxx.log files were generated in the current working directory