Former Enron chief executive Jeffrey Skilling will face questioning before Congress with other Enron officials whose versions of events conflict with his.
Sherron Watkins is also testifying. She is the vice president who warned former chairman Kenneth Lay in August of potentially serious accounting problems.
Mr Skilling has stated he knew few details of questionable transactions involving partnerships used to hide more than $1bn (around €716m) in debt.
The Senate Commerce Committee want to bring together Skilling, Watkins and Jeffrey McMahon, Enron's current president and chief operating officer.
Mr Skilling's lawyer Bruce Hiler says the former Enron boss specifically requested to be on the same panel as Ms Watkins.
"He is looking forward to what will be Mr. Skilling's first specific conversation with Ms. Watkins about these matters," said Mr Hiler.
But Ms Watkins' attorney, Philip Hilder, replied: "Mr Skilling ought to know that this is a congressional hearing, not a conversation with Sherron Watkins, and he needs to answer the questions and have the conversation with the congressional committee."
All three testified, separately and under oath, this month before a House investigative panel. Other top Enron executives, including Mr Lay, have invoked their Fifth Amendment right against potential self-incrimination and refused to testify.
Watkins told the House panel on February 14 she believes Skilling and Fastow - along with Enron's auditing firm, Arthur Andersen LLP, and its outside legal advisers - "did dupe Ken Lay and the board".
According to Mr McMahon's February 7 testimony, he was transferred out of his job as treasurer shortly after he complained to Mr Skilling about the partnerships in a meeting on March 16, 2000.