It seems a bit nonsence to stack this subject but this way you can use a wide aperture, good resolution and still get all the important features in focus and retain a nice blurry background. It is not a very colorful or beautiful picture but it is not a pleasant scene.
I first thought they were mating but when you look at the tips of their abdomen they seem similar. I don't think there are gay flies so I assume these are two males fighting and it could even be cannibalism. On this image it is not clear if the proboscis is piercing. Perhaps it is visible on some of the other images I took. I have to see.
Below a cropped part of the image. 50%
Wim

