Zaid tells ABC News' Chief Anchor George Stephanopoulos that the second person -- also described as an intelligence official -- has first-hand knowledge of some of the allegations outlined in the original complaint and has been interviewed by the head of the intelligence community's internal watchdog office, Michael Atkinson.
The existence of a second whistleblower -- particularly one who can speak directly about events involving the president related to conversations involving Ukraine -- could undercut Trump's repeated insistence that the original complaint, released on Sept. 26, was "totally inaccurate."
That original seven-page complaint alleged that Trump pushed a foreign power to investigate his political rival, Joe Biden, and Biden's son, Hunter, and that unnamed senior White House officials then tried to "lock down" all records of the phone call.
"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the first whistleblower stated, in a complaint filed Aug. 12.
Zaid says both officials have full protection of the law intended to protect whistleblowers from being fired in retaliation. While this second official has spoken with the IG -- the internal watchdog office created to handle complaints -- this person has not communicated yet with the congressional committees conducting the investigation.
The New York Times on Friday cited anonymous sources in reporting that a second intelligence official was weighing whether to file his own formal complaint and testify to Congress. Zaid says he does not know if the second whistleblower he represents is the person identified in the Times report.
Zaid’s co-counsel, Andrew Bakaj, confirmed in a tweet Sunday that the firm is representing "multiple whistleblowers." Zaid later confirmed in a tweet that two are being represented by their legal team.
According to the first whistleblower, more than a half a dozen U.S. officials have information relevant to the investigation -- suggesting the probe could widen even further.
Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee, made brief remarks Sunday afternoon about the second whistleblower at a screening of the film "The Great Hack" in Los Angeles.
"Let me just say this, with respect to the most recent development, and that is the public reporting of the second whistleblower stepping forward, that we are tremendously dependent on people of courage, stepping forward," Schiff said.
"Our very democracy depends on people of good faith and courage stepping forward to expose wrongdoing," he continued. "So the only thing I wish to say today about whistleblowers is we thank them for their courage. We thank them for their patriotism, and we hope others will follow their courageous example."
A transcript released by the White House of Trump's July 25 call with Ukraine President Volodymyr Zelenskiy showed Trump asking a "favor" of the foreign leader and pushing him to launch an investigation into the Biden family. Hunter Biden was on the board of a Ukraine energy company while his father Vice President Biden led policy on Ukraine during the Obama administration, leading some to question whether there was a conflict of interest or impropriety.
"There's a lot of talk about Biden's son," Trump told Zelenskiy at one point, offering the assistance of his attorney general. He later adds "a lot of people want to find out about that so whatever you can do with the attorney general would be great."
The White House cautioned that the transcript was not verbatim.
It is illegal for anyone to receive something of value from a foreign national in connection with a U.S. election, according to the Federal Election Commission. While it is not immediately clear whether Trump or other U.S. officials broke the law in its handling of Ukraine, that might not matter. The Constitution allows for Congress to decide what constitutes an impeachable offense.
Trump has denied any wrongdoing, calling the phone call "perfect."
"Like every American, I deserve to meet my accuser, especially when this accuser, the so-called "Whistleblower," represented a perfect conversation with a foreign leader in a totally inaccurate and fraudulent way," Trump tweeted Sept. 29.
White House press secretary Stephanie Grisham told ABC News in a statement on Sunday afternoon, "It doesn't matter how many people decide to call themselves whistleblowers about the same telephone call -- a call the president already made public -- it doesn't change the fact that he's has done nothing wrong."
ABC News' Zohreen Shah contributed to this report.