Over five hours yesterday, discussions between the jury and Wood, acting intermediary for the trustees, went back and forth.
The jury won the day. Ms Wood insisted she was resigned, rather than angry, about the "gesture".
She said: "There was nothing unclear about my communication of the rules but that was the choice they made and you have to respect it."
She added previous juries had entertained the idea of splitting the prize, but accepted it was not allowed. It was the most dramatic act of Booker insurrection since Philip Larkin, the 1976 chairman, threatened to jump out of the window if his favourite book didn't win (it did so he didn't).
Florence, director of the Hay Festival, said Extinction Rebellion protesters massed in the streets close to the Guildhall had provided inspiration.
"Today of all days, when rebellion is in the air - maybe we were a little moved by that," he said.
"We came down to a discussion with the director about the rules. And we were told quite firmly that the rules state that you can only have one winner. And as we have managed the jury all the way through on the principle of consensus, our consensus was that it was our decision to flout the rules and divide this year's prize to celebrate two winners."
Rather than some jurors favouring one book and some rooting for the other, all were unanimous in their desire to see both take the prize, he said.
The prize has been split twice before - between Nadine Gordimer and Stanley Middleton in 1974; and Michael Ondaatje and Barry Unsworth in 1992. Feeling a shared award detracted from the books - one better-remembered than the other - the rules were changed in 1993 to require one winner only.
Atwood's novel is a long-awaited sequel to The Handmaid's Tale and is comfortably the bestselling hardback of 2019, shifting close to 180,000 copies since its release last month.
Evaristo's novel sold a modest 3,976 copies. She is the first British woman of colour to win the prize and her book tells 12 interlinked stories about black women across the generations.
Florence said he hoped neither author would mind coming away with a £25,000 ($50,000) cheque rather than £50,000.
"I don't think its power and value resides solely in the cash," he said.
He also rejected the idea that having to share the prize with one of the world's most famous authors would overshadow Evaristo's achievement.