Culpepper has a better quarterback rating (112.2) than McCown (80.3) and more touchdown throws (3-0), while McCown holds the edge in completion percentage (54.3-53.6) and passing yards (277-200).
"It's become a two-man race," Kiffin said. "We'll let both guys play in this game. I don't have the exact reps (but) we'll play it by ear. We're still going to evaluate the two of them for the starting job."
McCown and Culpepper split reps with Oakland's first offense in practice on Monday and will continue to do so the remainder of the week leading up to the preseason game against the Seahawks.
McCown gets the start against Seattle while Culpepper will work in relief. McCown will be the only one who gets to work with Oakland's starting offensive line and receivers, however, as the Raiders plan to rest the majority of key players in their final preseason game.
"Those two have gone first, second, or third in games so they've played a number of reps behind the second line or with the second receivers, tight ends and stuff," Kiffin said. "It's no different as far as what you're evaluating. You're going to take all the information into account."
Culpepper chose not to discuss the situation with reporters, leaving McCown to summarize the two players' feelings.
"I don't think anybody's played a perfect game yet," McCown said. "We just try to improve and make that better and do the things that you do well, and hopefully make it an easy choice one way or the other. I think that's what every guy is trying to do that's involved in this situation."
The odd man out is Andrew Walter. Walter started against San Francisco on Aug. 18 but passed for only 154 yards and had a rating of just 69.5 in the preseason.
"This is not a reflection of Andrew playing poor," Kiffin said. "This is a reflection of the other guys, in my opinion, putting us in a better position to win."