Mark Zaid told ABC News the second person was also an intelligence official and they had spoken to the inspector general.
The White House has made no direct comment. President Trump has repeatedly rejected the original complaint.
No details have been released about the second whistleblower's claims.However, Mr Zaid said the person had first-hand knowledge of allegations associated with the phone call Mr Trump made with Ukrainian President Volodymyr Zelensky on 25 July.
The impeachment inquiry stems from the call, which was flagged up by the initial whistleblower in August.
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