Orwell Visits Miller At Villa Seurat
The two main published sources of information regarding this visit are:
 Alfred Perles in My Friend, Henry Miller (Part III, Chapter 7). Perles gives a first-hand account of the visit, having apparently been there at the time;
 George Orwell in Inside The Whale (beginning of Chapter 3).
According to D.J. Taylor's Orwell: The Life, Orwell "probably set out on 23 December." (p.200). He enlsited with the POUM militia on December 30th (ref) which he had done almost immediately upon arrival in Spain, so a visit to Miller probably occured somewhere between Dec 24 - Dec 28, 1936.
"When I had passed through Paris on my way to Spain it had seemed to me decayed and gloomy, very different from the Paris I had known eight years earlier, when living was cheap and Hitler was not heard of." [Orwell,  Homage To Catalonia, p. 220 - Penguin paperback, 1968 (full text here)]. According to Dearborn's Happiest Man Alive, Orwell came to Paris to "collect some travel documents" (p. 191).
Miller and Orwell engaged in a conversation that afternoon that seems to have revolved around their differences of opinion on whether one should fight for Peace (Orwell) or take a pacifist position for Peace (Miller): "Both were peace-loving men but, whereas Miller manifested his love of peace by refuing to fight for any cause, Orwell has no reluctance to engage in war, if the cause ere, in his opinion, a just one." 
Orwell writes that Miller had "no interest in the Spanish war whatsoever. He merely told me in forceable terms that to go to Spain at that moment was the act of an idiot"  (especially if one acted out of a "sense of obligation"). Orwell paraphrases Miller's countering thoughts: "Our civilization was destined to be swept away and replaced by something so different that we scarcely regard it as human" .
According to Perles, however, "Miller did not try, of course, to win Orwell over to his way of thinking or even to disuade him from going to Spain."  Orwell explained (acc. to Perles) how an "indelible mark"  was left on him from his unpleasant experience with the police force in India and all of the suffering he had witnessed. Miller understood how this gnawing guilt motivated Orwell, but wondered why he "chose to punish himself still further"  by going to Spain, when, in Miller's eyes, he had already suffered enough.
"To this Orwell made the classic reply that in such momentus situations, where the rights and the very existence of a whole people are at stake, there could be no thought of avoiding self-sacrifice. He spoke his convictions so earnestly and humbly that Miller desisted from further argument and promptly gave him his blessings." 
Before Orwell left to catch his train from the Gare d'Austerlitz to the Spanish front, Miller offered him a corduroy jacket. "'I can't let you go to war in this beautiful Savile Row suit of yours. Here, let me present you with this corduroy jacket, it's just what you need. It isn't bullet-proof but at least it'll keep you warm. Take it, if you like, as my contribution to the Spanish republican cause.'"  These are the formal words that Perles put into Henry's mouth; in an interview with Kenneth Turan in 1977, Miller quotes himself this way: "Change that coat, you're going to war, let me give you one of my old corduroys or something." Orwell accepted the coat but corrected Miller by telling him that his suit was actually from Charing Cross Road. 
As it turns out, the militia that Orwell would soon join wore corduroy breeches  (p.12).
This entire anecdote was written about in less detail in The Atlantic Monthly in 1995.
Miller and Orwell continued to stay in touch for the next decade or so. According to Miller in an interview with George Wickes in 1962, he met Orwell "maybe two or three times on [Orwell's] visits to Paris." There is no other record that I know of regarding a second visit.