[walrus01]

I understand that you want to do that, but any attempt to do so will be decidedly non-optimal due to how keyframes and lossy encoders work.

Even if your two files were encoded with x265 at exactly the same bitrate. It's a much more complicated problem than it appears at first glance, once you really dig into the command line options and encoding parameters of codecs like x264, x265 and vp9.

It's not as simple as concatenating two files together. You can also select down to per-frame precision using kdenlive and loading different x264,x265,vp8,vp9 files into it and cutting/editing them together. You will then need to re-encode the resulting output. kdenlive is ultimately a nice GUI front end on top of this:

https://www.mltframework.org/