A very thorough explanation, for which I thank you.
In my own defence, the first response contained the phrase 'fairly certain' - which allows for error, and the second linked to a highly informative article which nevertheless did not make clear to me the concept that two aircraft could occupy the same track at a horizontal as well as vertical distance, although with 60 miles between each one I suppose it's inevitable that such freedom is available.
I've merely taken the opportunity to compare what I saw with the offered explanations of how things work, maybe discuss it a little. I certainly don't mean to question anyones knowledge or advice; my last sentence said virtually the same thing your last sentence did - that I do not fully understand, but will nevertheless accept that the occurrence was probably normal.
As it happens the explanation you have given me is a very enlightening and pertinent one, and answers my questions thoroughly. The tone of it though would make me think twice before asking another one.