I have a video clip in track "Video 1", and another clip directly above in track "Video 2". I'm not sure the correct terminology, but I want to set the "opacity" of the clip in upper track (Video 2) to about 50% so that the two video clips merge/dissolve into each other.

This would be similar to the "Dissolve" transition, except that I want the effect to stay consistent throughout the entire clip (not a transition between two clips on the same track, rather a "dissolving" of two full clips into each other).

Hope that makes sense, and that someone can help.

Thanks in advance for an input.

(Kdenlive 18.12.3 on Debian)

you can use the "transform" effect and set opacity there. You can even use keyframes to set different values at different times.

Many other effect I tried have a problem with fading effects, e.g. borders around an image are shown are somehow black areas or shadows show up etc. "Transform" is the effect that works for me very well.