These things can be intensely frustrating sometimes.
I sense you are 'close' - but then that doesnt help does it?
What sort of video files are they by the way? mp4, mkv, avi?
I seem to recall there was a way you could go to the PC that isnt running serviio and open a web browser. - You could type a special web address and should be able to stream to the second PC. Effectively the second PC would be a DLNA client - would be good for testing. See if you can still do that - maybe in the help pages.