In 2011, Microsoft assigned about 55 percent of its profits to the subsidiaries in Ireland, Singapore and Puerto Rico, said Stephen Shay, a tax law professor at Harvard University. Shay and two other tax experts also questioned the Microsoft and HP methods.

HP uses a different mechanism to avoid U.S. taxes, Levin said. U.S. tax law requires companies to pay U.S. taxes on foreign profits sent back to the U.S., but there's an exception for short-term loans. In recent years, HP subsidiaries in Belgium and the Cayman Islands have provided a near-constant stream of short-term loans to HP in the U.S. to pay for a variety of expenses, including U.S. payroll and shareholder dividends, Levin said.

In response to repeated questions from Levin, Bill Sample, Microsoft's corporate vice president for worldwide tax, and Lester Ezrati, HP's senior vice president and tax director, both acknowledged that their companies considered the tax implications when setting up the programs in question.

But both also stressed they were operating within the current U.S. tax code.

"Microsoft complies with the tax rules in each jurisdiction in which it operates and pays billions of dollars each year in total taxes, including U.S. federal, state, and local taxes and foreign taxes," Sample said.