For brainlearn-like engines, the "concurrent experience" option asks each instance of the engine to use an unique experience file, even on the same directory. So at startup, each instance adds few random characters to the filename of their copy of experience.bin file, e.g. experience-d5h3z.bin. So there is no concurrent access on an unique experience.bin file even with concurrency=1000 on the GUI. At the end, the user runs brainlearn which will automatically merge all experience-*.bin files into an unique experience.bin file. That's all !
As there is no such option with Eman, each time you see or heard about a concurrency > 1, the tester is unenlighted. At best he got few crashes and his eman.exp doesn't contain all played moves from each instance. At worst, his tourney was interrupted after tens of crashes because several instances tried to read/write on the same and unique eman.exp.
it's been years 
256 threads 
