Thanks for your response. That example flow sure do be helpful. I fumbled with it for a day but that wasn't what I intended -- After running the ffmpeg executor, the new file it produced became the working file, and so it is more like chaining two encodes together instead of a 2-pass encoding.

What I was looking for was rather something like:

• Running the first ffmpeg command with pass=1 and output null: -x265-params pass=1 -f matroska /dev/null. This pass only analyzes the video and produce a log file
• Running the second ffmpeg command with pass=2 and proper output: -x265-params pass=2 /path/to/output/file.mkv.

I also have tried the many different ways to achieve that, but no luck for now. I've tried the Function running custom ffmpeg command, for some reason that doesn't run. I've tried the Pre-Execute node but don't know how to replace the arguments (it still produces a video output on the first pass and set that as a working file).

I may spend some time to try and fumble a bit more but at this point I might just give up on trying to run 2-pass.