What a rare treat this sale is – and what an opportunity. Furniture, art and wonderful objets from the private collections of Gordon Watson and Hamish Bowles, two of the best, most eclectic and most passionate collectors of our time – both famous, though quite different, tastemakers, both armed with huge amounts of knowledge, both with an impeccable eye and both with a gift for unearthing treasures wherever they go.

You may know Watson's exquisite taste and style if you have visited his jewel-box of a shop on London's Pimlico Road, famous among designers and decorators as one of the first places to sell 20th-century furniture and pieces by designers and craftsmen such as Giò Ponti, Jacques Adnet, Angelo Mangiarotti and Ado Chale – long before such names became more widely fashionable. In Britain, Watson has also become well known after two series of the hit TV show about antiques and curiosities, Four Rooms. Watson is so good on it, and so popular, that it is no surprise to hear that he is currently in talks to film an antiques show in the US with co-host Whoopi Goldberg.

Likewise, the British journalist, writer, curator and Vogue's International Editor-at-Large, Hamish Bowles needs little introduction. His apartments in Paris, London and Manhattan have been photographed frequently for magazines such as World of Interiors, and like Watson (the two have been friends for 25 years), his taste is legendary. He is also known as one of the foremost collectors of 20th-century haute couture, and has curated exhibitions at the Metropolitan Museum of Art's Costume Institute (Jacqueline Kennedy: The White House Years) and, most recently, Balenciaga: Spanish Master at the Queen Sofia Spanish Institute in Manhattan and Balenciaga and Spain at the de Young Museum in San Francisco.

As well as being one of the most respected authorities on fashion, Bowles has written about the interiors of the world's great tastemakers for more than 25 years. And wherever he has gone, whatever he has done, he has collected. "I first started going to antiques shops as a little boy with my mother," he recalls. "If we were driving anywhere, we'd shriek at the top of our voices if we spotted somewhere inviting, then leap out of the car. My collecting really has been a life-long passion, particularly for finding things unnoted and undetected."

It is this passion for the hunt that unites Watson and Bowles. Watson is a dealer by profession, but the way he buys has only ever been personal. "My passion is to discover, find and buy," says Watson. "I've always only ever bought things that I love, that I want to live with myself. Economics means I have to sell." And so here he is, selling pieces he has loved and lived with, either for years or acquired more recently. There are four pieces by Angelo Mangiarotti, for example, the Italian designer Watson has been obsessed by since, 30 years ago, he first saw a marble table from the designer's 'Eros' collection, which belonged to the contemporary art dealer Lewis Kaplan. "Years later, I started to see Mangiarotti's pieces in other places, and realized it was the same person who'd made the table I'd loved so much. I bought everything I could. The two unique vases in this sale are the last of eight I bought years ago. The architect Norman Foster bought the other six from me. I'm a dealer so I have to sell, but I have a feeling these will be among the things I will miss most."

If Mangiarotti has been a life-long love, Watson is no less enthusiastic about more recent discoveries. "That's what I'm most interested in now," he says. "The contemporary craftsmen who are making the antiques of the future." Such as? "Paul Belvoir," Watson shoots back (this is the first time Belvoir's sought-after rock crystal and bronze lamps will be available to buy). "Studio Job. After I saw an exhibition of their work, I rushed to their studio and bought a lacquered pedestal inlaid with laser-cut skulls and an extraordinary dining table. I've had both in my London flat since, but they're so beautiful they need more space, more stillness. I live in a bustling converted chapel and have too many parties."

As well as contemporary pieces and 20th-century furniture, Watson has collected, with zeal, Indian and Islamic objects, contemporary art, Moroccan textiles, and embroidery from Madeira, among other things. Not that one passion ever replaces another. Instead, it is typical of Watson's eclectic style to layer pieces from different cultures and periods and mix them together. So you might see a pair of Belvoir's singularly beautiful contemporary rock crystal and silver objets sitting on a pair of 1940s Jansen ebonised console tables, which themselves are in front of a 2001 photograph by Malerie Marder.

This enviable way of mixing pieces to such heightened and glorious effect is not common, but if anyone can match Watson in this art, it is Bowles. Anyone who has had the pleasure of walking into one of Bowles' homes will have seen something they'd sorely love to tuck under their arm and take away to live with. The pieces in this sale – furniture, works on paper, and objets from the 18th to the late-20th century – are these very things. So yes, here is the series of drypoint etchings by French artist and illustrator Etienne Drian that hung in Bowles' Paris drawing room, featuring actresses, courtesans and society beauties of the first half of the 20th century, which Bowles has collected since buying his first from a Soho print shop in London when he was a student at St. Martin's College in the 1980s (and paid for with his entire term's living grant, which meant eating only Ryvita crackers for the next three months). Here, too, is a 'Ram' chair, one of the first pieces made by André Dubreuil, the former trompe l'oeil painter, decorator and collector, who started designing and making his own furniture in the early 1980s, learning to weld while sharing a studio with Tom Dixon in All Saints Road, close to Bowles' first London flat.

Here you will find pieces from Bowles' craze for 1930s neo-Rococo revival decoration (in particular, wonderful Syrie Maugham and Serge Roche work), next to pieces made by Karl Springer in New York in the 1970s, another era Bowles adores, and, say, an 18th-century chair (such was Bowles' passion for finding and buying the latter that he began to have to stack them up rather than sit on them), behind which might hang a photograph by Wolfgang Tillmans or an etching of a weed growing though a crack in a London pavement by British artist Michael Landy. This is the way Bowles has always decorated, as he has lived, finding things (places and people) he loves and mixing them together.

"Things from very different eras can create a dialog that can be exciting and energizing," explains Bowles, who for the first time has collaborated on the interior of his new Manhattan apartment, with the Milan-based decorators Roberto Peregalli and Laura Sartori Rimini, an experience he describes as "completely fascinating and salutary. I admire them so much. What they've done is to take my spirit and create an extraordinary framework, but they also have a vigorous esthetic, which has meant that we've had to do an edit of all the pieces I've found from the lives I've lived, and sadly... Sadly," he continues, his voice a combination of excitement and mournfulness, "there just isn't room for everything." Sad for him, maybe. Sad for Watson that he's selling his last beloved Mangiarotti pieces, his most cherished Belvoir objets. But great news for us.

Daisy Garnett writes for Vogue, The New York Times and The Daily Telegraph.

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