Byrnes had his first multi-homer game in two years and drove in five runs and Young also homered while scoring four times Friday, leading the Diamondbacks to a 12-3 victory.
"It's very important for me to finish the season strong. Same for C.Y.," said Byrnes, whose previous multihomer game was May 30, 2007. The five RBIs fell one short of his career high set July 4, 2004.
"Both of us have had success in this league and we know we can have success again."
Byrnes and Young had four hits apiece, and it marked only the second time all season the outfielders homered in the same game. The other was April 21.
In 2007, Byrnes batted .286 with 21 homers and 83 RBIs while Young, a rookie, had 32 homers and 68 RBIs. Each homered in the Game 3 clincher at Wrigley Field.
The Diamondbacks lost their next series to Colorado but expected to be strong for years. Instead, they missed the playoffs in 2008 and are in last place at 69-91 this season.
Byrnes missed two months this year with a broken hand and is batting .226 with eight homers and 31 RBIs. Young is hitting .214 with 14 homers and 39 RBIs. Both spent part of August at Triple-A Reno, Byrnes on a rehabilitation assignment and Young trying to find his hitting stroke.
"When we were down in Reno together, that was as much fun as I've had in two years," said Byrnes, who missed most of 2008 with hamstring injuries. "It took that Reno experience to realize how much I love the game, because a lot of that love was lost the past two years. Being injured and scuffling and external factors ... made baseball not fun."
Young's four hits Friday were as many as he had in the previous six games combined.
"We're just focusing on this series right now to take something good into the offseason," he said.
This also has been a disappointing season for the Cubs, who were expected to win a third consecutive NL Central title but instead are 82-77 and headed to a second-place finish behind St. Louis.
