Justine Greening and Phillip Lee say Mrs May has ignored requests from pro-EU Tory MPs in favour of Brexiteers.
The pair had separate meetings with the PM in Downing Street.
Ms Greening said she had considered joining the three MPs who quit the Conservative Party on Wednesday – and would do so if there was a no-deal Brexit.
The defectors, like Ms Greening and Mr Lee, back another EU referendum – and they say more Tory and Labour MPs are poised to join their new Independent Group.
Ex-Tory MP Heidi Allen told ITV’s Peston programme “a third” of Tory MPs were fed up with the party’s direction.
Ms Greening and Mr Lee, who quit as a justice minister over Brexit, have been named by Ms Allen as a potential future defectors to the Independent Group.
The Right to Vote group, which is chaired by Mr Lee, said he had discussed the campaign’s calls for a pause in the Brexit process and a possible second referendum with Mrs May.
“Talks were open and we are encouraged she listened to our case,” the group said.
Mr Lee has said one of the reasons the Tory MPs had decided to quit the party was the access the Brexiteer European Research Group got to the prime minister, who he said had refused to meet his wing of the party.
Justine Greening – a former education secretary – told the Today programme she had been tempted to break away from the Conservative Party and join the Independent Group.
“It is something that I have considered, but I have reached a different conclusion for the moment,” Ms Greening told Today.
“I don’t think I would be able to stay part of a party that was simply a Brexit party that had crashed us out of the European Union.”
The Independent Group was set up by eight defecting Labour MPs unhappy about their party’s handling of Brexit and anti-Semitism.
They were later joined by three pro-Remain Tories – who accuse the Conservative leadership of allowing right-wing hardliners to shape the party’s approach to Brexit and other matters.
Labour’s Ian Austin also expressed sympathy with the Independent Group’s aims, saying he would think “long and hard” about his future in the Labour party.
Shadow home Secretary Diane Abbott told BBC Radio 4’s The World at One: “I am very sad that the Labour members of this new independent organisation have gone.
“Up until the last minute, people were talking to them, trying to persuade them not to take the step they have taken.”
She said she hoped they would continue to work with Labour on issues like homelessness, the benefit system, the NHS and “most of all fighting this Tory Brexit”
