Two aircraft flying close to one another and less than what ever the minimum separation distances are in Part 91 these days.
I doubt that Part 91 would allow for low level air racing in high performance aircraft without an expensive CASA approval. If not, let me know and I’ll get a few mates together and get stuck into it like horse racing! I’ll get betting mobs and the TV stations involved that will immediately give me a ticket to retirement 😀
Now that would be an aviation success story👍👍👍