When Joan Rivers was a 25-year-old aspiring actress who had yet to change her name from Joan Molinsky she played in an off-off (and possibly off) Broadway play with another unknown called Barbara Streisand who was then 17-years-old.

Rivers’ own version of the story in her autobiography describes a production in which she played a lesbian who attempts to stab Streisand’s character during a love scene. With a cast like that who would not want to time travel back to the late 1950s and buy a ticket for that play?

Well, it turns that if you could — and did — you might be disappointed. The play, not called Seaweed as Rivers remembered but Driftwood, had no such scene or even a lesbian character. But Joan and Barbara were photographed together in rehearsal.

And that would have been the end of it had playwright Roy Smiles — a specialist in reviving lost historical moments, and if necessary the people who were there — not written a “what if” comedy that imagines what conversation passed between the two nascent mega stars. (Although the comedian Sue Perkins did write a TV short for Sky)

“I spent two years getting depressed writing my Kurt Cobain play and I really needed something to lighten up life,” says Smiles on Zoom. He is joined by Rosanna Harris (who plays Streisand) and Mia Tomlinson (Rivers) from their respective homes.

“I’ve been in love with Streisand since I was about nine,” continues Smiles, “when I saw [the movie] The Owl and The Pussycat. And I’ve always loved Rivers. And my first play was about Lenny Bruce who was Joan’s guru, so it seems like a cycle of life really.”

“As soon as the possibility of playing Joan Rivers came up I was like ‘I wanted to do it’ says Tomlinson who is best known for dramas The Beast Must Talk and the Netflix swashbuckler The Lost Pirate Kingdom.

“It’s been drilled into us that we are not doing an impersonation,” says Harris. “So it’s down to us to emulate them and get their vibe and charisma.”

For Smiles, the chance to write female roles was one motivation.

“I thought why aren’t I writing more female characters? And because I’ve loved Jewish comedy since I was a little kid — I adored Danny Kaye, Jerry Lewis, the Marx Brothers, and Jack Benny — I just thought why not do it about wisecracking New York Jewish women? And Streisand and Joan Rivers are just obvious.”

On the page the resulting dialogue is brimful of one liners which by the second act “becomes a kind of power play.” After the off-Broadway venue the second act opens in Vegas where both women have their name in lights.

Harris characterises the verbal sparring as “dig, dig, dig, dig, dig … but I love you, and if you need me I’m there.”

Smiles learnt a lot about his heroines while writing the play.

“I just had no idea how poor Streisand’s background was, how threadbare it was, sleeping on couches until she was 18 or 19. It just shocked me, because she never really talks about it. With Joan I learned how she cracking gags about her husband’s suicide the day after he died. She was just she brave and unafraid of what people think. So yeah, there were a lot of surprises during the research.”

Of the three, Harris is the only bonafide Jew, but “My brother went to a Jewish school,” says Tomlinson and Smiles adds “I lived in Israel for a year, so I’m a token Jewish person.”

“And I’m handing out the tokens,” says Harris. “Rosanna is also in charge of pronunciation of the Yiddish words in the play,” adds Tomlinson.

“My mum has every song, every album by Streisand. Whenever I got into the car when she picked me up from school Streisand. So when I told her I’m going to play her — she’s very excited. She’s bought a lot of tickets. I think she’s bringing the whole of north west London, ” says Harris.

They each have their Jewish comedy inspirations. “For me it’s Larry David and Curb Your Enthusiasm,” says Tomlinson

“I just don’t think it ever ages really,” says Smiles. I can watch the Marx brothers all night and I’ve just re-watched Seinfeld for the fifth time because I’ve been indoors and watching a lot of box sets. I just think it is fantastic strain of humour. He has been put on Netflix and it is going to get millions of hits. I just don’t think it is going away.”

Harris however has a slightly surprising comedy choice — Streisand herself. “I straightaway think of Barbra,” she says. “That’s the bottom line. She is in my eyes the most iconic Jewish legend that there is, apart from her incredible voice, which is second to none, there is her acting skill and comedic scope.”

And Tomlinson raves about Rivers .“I’ve become a massive fan through acting her. It’s part of the job to find empathy and love but with it really has been easy to fall in love with Joan. I found out today she was the first female comic that was a regular on a TV. And she did it pregnant as well. You learn a lot about how to laugh through pain.”

The Funny Girls is The Studio New Wimbledon Theatre until September 24, and then upstairs at The Gatehouse from Oct 26 to Nov. 21.