My buddy and I have been flying together for the last couple days- we call out our altitudes during our flights and also call out if the quads are incoming towards us or outgoing away from us. It’s all about communication.
That was in the days of AM / FM radios and crystals. We all went through the years of Peg Boards / Frequency Control. Flags on antenna to denote channel colour / number so others knew what you were on.
Todays frequency hopping radios have put all that to the past.
I am involved in crash testing from time to time. I’ve seen two phantoms and a mavic flying at the same time in the same area for different views. Don’t know it it was supposed to be done but there were no issues.