Piercy shot a 67 to go with his opening round 62 and McGirt added a 66 to his 63 as the pair finished the second day tied for the lead on 11 under.
But it was a bad day for England's Greg Owen who fell out of contention, shooting two over for the day and ending it on five under in a tie for 17th at the Hamilton G&CC in Ontario.
There was also bad news for home player Mike Weir who missed the cut for an 11th straight start.
McGirt admitted: "I've never been in this position out here.
"I was in the next to last group at Tampa this year. It was a lot of fun. I'm looking forward to it."
Americans Robert Garrigus and Bo Van Pelt lie third and fourth on 10 and nine under respectively, with Fiji's Vijay Singh tied for fifth on eight under with Clark and American Josh Teater.
Clark put his 62 down to the soft ground conditions.
He said: "Obviously being soft you're able to go at a lot of flags and make a lot of birdies."
Following wrist surgery in 2010 Clark had seen his form suffer and in his first six starts this season he missed the cut four times and was disqualified from another event.
He added: "The only way to get better was to come out here and play tournament golf.
"I could feel I was getting better every round I played."