Last year, Big Mac and Slammin' Sammy homered on the same day 21 times as McGwire finished with a record 70 and Sosa also broke Roger Maris' 37-year-old record with 66.
"Last year was something else," Sosa said. "This year has been great, but it's not like last year. But we're there again, and who knows what can happen?"
Cubs manager Jim Riggleman, for one, is worried more about job security than another home run competition.
"I can't tell you how uninterested I am in that," Riggleman said. "The significance of (Sosa's) home runs is they help us win ballgames, not put himself in the record books."
With a sellout crowd of 48,615 on its feet in the third, McGwire homered for the third time in two games. His homer off Cubs rookie left-hander Micah Bowie tied Eddie Murray for 15th place on the career list at 504 and hiked his NL-leading RBI total to 110.
"You talk to me a half-hour after the game, I don't have any thoughts," McGwire said of tying Murray. "Obviously, it's awesome."
Last year, McGwire also hit No. 47 in the Cardinals' 118th game. He hit his 48th and 49th homers in the 124th game against the Cubs.
Sosa, who entered the game in a 3-for-31 slump, hit a towering drive to eft on a 2-0 pitch from Darren Oliver with two outs in the seventh to put the Cubs ahead 7-5.
"That was a dumb mistake by me," Oliver said. "I'll see it about 20 times on SportsCenter, so I'll try not to watch TV tonight."
Sosa also singled and scored in a four-run first and reached 100 RBIs for the fifth straight season, tying the team record set by Hack Wilson from 1926-30.
"Everybody that plays this game goes through those kinds of slumps," Sosa said. "If I don't feel good at home plate, I swing at everything. But when you believe in yourself and you know what you can do, that will go away."
Glenallen Hill's RBI grounder off Heathcliff Slocumb (3-2) with the bases loaded in the ninth snapped a tie as the Cubs ended a six-game losing streak, their longest of the season. Chicago also snapped an eight-game losing streak at Busch Stadium dating to Sept. 26, 1997.
Henry Rodriguez added an RBI single in the ninth to extend his hitting streak to 15 games. Cardinals catcher Eli Marrero contributed to the inning when he threw wildly to first Juan Nieves' sacrifice to put runners on second and third with none out.
"As soon as I let it go, I knew it would be a terrible throw," Marrero said.
Terry Adams (4-2), the fifth Chicago pitcher, pitched two perfect innings and struck out three, including McGwire to end a 1-2-3 eighth.
Hill also had a two-run double, and Mark Grace and Gary Gaetti had run-scoring singles in a four-run first for the Cubs. McGwire's homer cut the gap to one and Craig Paquette also homered in the third as the Cardinals tied it.
Edgar Renteria's RBI single in the sixth gave the Cardinals a 5-4 lead. The Cardinals tied it at 7 in the seventh on Paquette's RBI single and a balk by reliever Scott Sanders, which brought Ray Lankford in from third.
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