Please don't take me too seriously, I'm being deliberately controversial
I was around during the C64's golden years and I can't remember ever hearing anyone do the single-voice-echo trick on that platform back then. I never had a Spectrum so I've no experience.
There was certainly no instance of me hearing a single-voice-echo on any game music and making a deliberate attempt to mimic it. Double-voice, yes., many times.
In actual fact, in Magician, I accidentally made the sound by having a software envelope that reduced rapidly from full volume to very low, say, an amplitude of 15 down to 2 so you had a sound with a very sharp attack but a quiet sustained portion that lasted until the next note. So I wasn't even playing two notes but that was what started it for me. I noticed the cool effect it made and then sought to exploit it by way of then placing a lower volume note after a loud note in the way we all now know.
In later titles I even worked out on paper how to have single-voice echo where the echo occurred not straight after the note but a couple of notes later. It was an utter bastard to edit though.
Now, in Nijuu, it's actually programmed into the engine so it's now utterly simple to achieve the effect and you can get a much wider range of echo effects by manipulating a couple of parameters.
I've got a simple demo that repeats a (well known) melody while changing the echo parameters which I can stick up on the website if anyone is curious.