The margin between the two is a wafer-thin 0.023 average points, the narrowest gap between the world's top two players since the official rankings were launched in 1986.
McIlroy and Spieth are both in the field for the BMW Championship, the penultimate FedExCup playoff event of the season, and either of them -- along with third-ranked Australian Jason Day -- could be world number one next week.
"It'll be like that until one of us separates ourselves a little bit," McIlroy told reporters ahead of Thursday's opening round at Conway Farms Golf Club in Lake Forest.
"At the end of the day, it's just about playing and playing well. I don't know any other way we could determine the best player in the world.
"You could do it on a one-year point system instead of two. I think two years is a good reflection of how you played."
The ranking system is structured on a two-year 'rolling' period with points awarded for each event and then maintained for a 13-week spell to give additional emphasis on recent performances.
Ranking points are then reduced in equal decrements for the remaining 91 weeks of the two-year time frame. Each player is ranked according to his average points per tournament.
"We're all focussed on our own goals," said Masters and U.S. Open champion Spieth, who became world number one for the first time after finishing runner-up to the triumphant Day in the PGA Championship at Whistling Straits last month.
"As one, two and three in the world, we're the three that have to beat each other at the top right now in order to try to get to the top, or to remain at the top (of the rankings).
"I'm not focussed on what either one's doing on the leaderboard unless they're in the lead, and then if they're in the lead, how do I get up there and surpass them?"
The only previous time when there were changes at the top of the rankings for four successive weeks was in June 1997 when Greg Norman, Tiger Woods, Ernie Els and then Norman again traded places.
Odds are good that McIlroy and Spieth will keep their 'merry-go-round' at the top going into a fifth consecutive week and perhaps beyond, with Day in close pursuit to make it a three-way battle.
Golf fans can now savour the tantalising prospect of that trio challenging for supremacy in the blue riband events for at least the next five years with McIlroy aged 26, Spieth 22 and Day 27.
Between them, they have won five of the last six major championships played with all three having developed a habit of shining at their brightest on the sport's biggest stage.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)
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