Two presidential hopefuls topping the Republican field, Donald Trump and Ben Carson, announced Thursday in a joint letter to CNBC they will not be participating in the third GOP debate if it is exceeds two hours and excludes opening and closing statements.

The outsider duo, first and second in the Washington Examiner power rankings, cited concerns that the second debate's three-hour length was too long and said personal statements serve to ensure each candidate has a chance to be heard.

But a top aide to one of Carson and Trump's rivals said perhaps there's a different reason they're calling for a time limit.

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Sarah Isgur Flores, deputy campaign manager to Carly Fiorina, suggested the two candidates might be avoiding a prolonged on-stage competition with Fiorina.

Asked by a reporter what she made of Carson and Trump's request for closing statements, Isgur Flores said she would prefer to see the candidates field additional questions.

In an earlier statement to the Washington Examiner, CNBC spokesman Brian Steel said the network plans to host "the most substantive debate possible," but would take Trump and Carson's suggestions into consideration.

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