Why would you feel the need for both, do you really see that much difference?
For me 24mm is normally as wide as I go with video that focuses on people or animals. After that I find that I spend, what I consider, too much time correcting distortion or trying to avoid the edge of the frame.
I also like using a fast 24mm for events and street photography because I feel like I can get the scene but also get some environmental portraits without switching to a 35, just keeps me moving more and changing lenses less. Ideally I would want a 24mm 1.2 or 1.4, but 1.8 works.
As for where the 20mm fits in, I primarily use it for landscape photography and in the city. I love the slight warping you get that can make a city seem towering and put a little more character into the straight lines of a rigid cityscape. It can also really make graffiti pop if you use it to frame your subject. For me 24mm just isn’t quite wide enough in a lot of cases but I don’t like the 16mm look as much. As for video I rarely use the 20mm, though I have used it in the past for filming skateboarding and a few shots in Denali National Park.
Obviously the lenses are still just a twinkle in the engineer’s eye at this point and the characteristics could differ from my expectations but I have access to a Sony A7III and can’t say enough good things about their 20mm 1.8 and 24mm 1.4 GM.