Kelly had eight birdies and a bogey at TPC Snoqualmie Ridge. The three-time PGA Tour winner is making his 13th senior start after turning 50 in November.
McKenzie eagles the par 5-eighth to join Kelly atop the leader board.
Jordan Spieth reeled off five straight birdies on the back nine for a 5-under 65, moving into a four-way tie for the lead with Dustin Johnson, Rickie Fowler and Jhonattan Vegas after two rounds of The Northern Trust in Old Westbury, New York.
Madison’s Steve Stricker had a 70 and missed the cut.
The top 100 players in the Fed Ex Cup point standings after the weekend, head to the second event in the chase for the championship next week in Boston.