Unusual ones? Well, Mervyn Peake co-wrote a book with famously camp wit-about-town Quentin Crisp, who became famous when John Hurt portrayed him in the 1970s. I suppose this feels weird since Peake feels like a man of the 50s, while Crisp came to prominence much later.
George Orwell knew HG Wells and was taught at school by Aldous Huxley. There's a letter from Huxley to Orwell effectively saying "Congratulations on your dystopia, but mine is even worse". Orwell also nearly had a fight with a drunken Earnest Hemmingway, which makes more sense given that they were both on the anti-fascist side in the Spanish Civil War.
Most times had a literary "scene" so you can expect a lot of artists to know each other through that (even those who wouldn't own up to it). There are certain moments, like the Spanish Civil War, that seem to squash different artists together with a common cause. Also you get talented people who lived in chaotic times that allowed them to run into all kinds of people, like the composer Joseph Bologne.