David Mcsween told me about the ghost button - also called the frame overlay function: https://docs.blender.org/manual/en/dev/editors/vse/preview/introduction.html
It allows one sequencer preview to be a one place in the timeline and another sequencer preview to be at an offset frame. This way it would be possible to view the last frame of the first clip to the left and the first frame of the second clip to the right when trimming.
And in theory it should be possible:
I wanted to try it out, the same way I did above with the instant preview when trimming, but I realized that the “continuous” functions in VSE Quick Functions have either been moved or removed? What has happened to those functions(running in modal mode)? Do you think this theory will work or not?