Charley Hoffman and Matthew Jones were one shot off the pace at 8-under-par 136. Erik Compton, who shared the opening-round lead with McKenzie, and Jason Schultz followed at 7-under-par 137.
Hearn played the back side first at Redwood Meadows and jumped out of the gate with birdies on each of his first two holes. Hearn then eagled the par-5 15th and added back-to-back birdies starting at the par-3 17th to move to minus-eight.
'I tried not to get ahead of myself,' said Hearn. 'I knew I had a lot of holes ahead where I could still make birdie so I was trying to just keep putting it in the fairway and hitting it close.'
The 25-year-old kept on cruising and picked up two straight birdies from the second to reach 10 under. Hearn then faltered to a bogey at the sixth but he responded well with a birdie at the par-five eighth. Hearn found trouble again at the ninth, however, and bogeyed the hole to fall back into a tie for first.
'Keeping the ball in the fairway is absolutely the most important thing out here,' said Hearn. 'Yesterday I hit only four or five fairways and that really limits your ability coming into the green to place the ball where you need to.'
McKenzie also started on the 10th tee but was not as fortunate as he bogeyed his first hole. He countered with a birdie at the 11th but stumbled at the very next hole with another bogey.
The Australian got back to even-par on his round with a birdie at the 14th and tallied a pair of birdies on the front side for a round of 70.
'I didn't have as many chances to make birdies as I did yesterday. That was probably the biggest thing,' said McKenzie. 'I didn't putt as well as I did the first day.'
Paul Gow, Jeff Klauk, John Morse, Kevin Johnson, Kelly Sellers and Keith Nolan tied for seventh place at 6-under-par 138.
The 36-hole cut fell at 1-under-par 143 with 63 players qualifying for the final two rounds. Among those who won't be around for the weekend was Brendan Jones, who is currently ranked second on the Nationwide Tour money list.