Originally Isaac Asimov's Robots and Foundation series were separate entities, but were later unified. The first book in which I remember the connection being made clear was Foundation and Earth (1986) in which
it transpires that R. Daneel Olivaw from the Robots series has lived for millenia, using his pyschic abilities to shape the course of human history. This is expanded upon in the Foundation prequels where it turns out he directly worked with Hari Seldon and was a driving force in the creation of the Foundation.
The original series were completed in the 1950's with Second Foundation (1953) and The Naked Sun (1957) being the last in each series. At this point there is clearly no connection between either series as far as I can tell
Nothing was then written in either series for 25 years until Foundations Edge (1982) and The Robots of Dawn (1983) restarted both series. It seems possible that Asimov at this point was intending to merge the series as several plot points that are important in Foundation and Earth are established.
Notably the introduction of a psychic robot in The Robots of Dawn and a whole planet linked by telepathy in Foundations Edge, which R. Daneel sees as a possible future evolution for the entire galaxy.
So my question is this: When did Isaac Asimov first decide to combine the two series? At some point in the 25 year years between 1957 and 1982? Or was it not in fact until after he had already restarted both series. Has he ever gone on record as to when this happened? Bonus points if he made any comments on why he decided to do this.