http://www.truthout.org/docs_02/11.28A.bush.no.911.htm]
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the Tower Commission, which investigated the Iran-Contra affair under President Reagan. The president testified in 1987 to the panel he had appointed to probe the actions of the National Security Council staff following disclosure that profits from secret arms sales to Iran were diverted to the Contra rebels in Nicaragua.
Fleischer also was reminded that President Ford testified before a House subcommittee Oct. 17, 1974, to explain his pardon of former President Richard Nixon. Ford had said he did not want to set any precedent, but thought the nation's interest outweighed presidential prerogatives during the Watergate scandal.
and this:
http://www.buffalonews.com/editorial/20040426/1037573.aspIt is common for congressional panels to have joint appearances by witnesses. But these are generally done at the request of the committees to save time or to encourage a debate among the witnesses. Experts in probes of the executive branch said they could not think of an arrangement similar to that required by Bush and Cheney. "Something like this may have happened sometime, somewhere; I just haven't heard about it," said John W. Nields Jr., who was chief counsel to the House committee that investigated the Iran-contra scandal during the 1980s.
Bruce Fein, who served in the Justice Department during the Reagan administration and on the Republican staff of the congressional Iran-contra investigation, said such an arrangement has both constitutional and investigative flaws.
"A joint appearance sabotages the idea of a unitary executive - the "buck stops here' attitude of (President) Harry Truman - by enabling Bush to shift blame or accountability to Cheney when politically expedient." In investigations such as Iran-contra, Fein said, "customary legal rules require sequestration of witnesses in depositions and at trial to avoid tailoring the truth to avoid inconsistencies."