Royals third baseman Mike Moustakas and Cardinals pitcher Carlos Martinez are among the five candidates for the final spot in their respective leagues. Fan voting will determine who makes the roster.
"While the Royals-Cardinals rivalry is among the best in the game, we are proud to join forces to urge fans to send a clear message that we are teams united in our commitment to ensure that the best players take the field for the All-Star Game," Royals president Dan Glass and Cardinals counterpart Bill DeWitt III said in a statement Wednesday.
The regional sports networks for each team are promoting the alliance, and Missouri Gov. Jay Nixon said Wednesday that fans should vote "so there is as much Royals blue and Cardinals red on the field next week in Cincinnati as possible."
The Royals already have six players on the American League roster, including four starters. The Cardinals have two starters and a total of five players on the National League team.
Both totals are the most in their respective leagues.
"I voted. But I kind of do the traditional vote, one time," Royals general manager Dayton Moore said before Wednesday night's game against Tampa Bay. "But I was told someone in my office is using my account so that all 35 votes are used."
Moustakas, who has been on the bereavement list since Monday, was beaten out in the initial fan vote by the Blue Jays' Josh Donaldson, but still had one of the highest totals of any player regardless of position. He is hitting .301 with seven homers and 31 RBIs.
Royals manager Ned Yost, who will handle the AL managerial duties, said he chose Moustakas for the final vote to reward him for revamping his swing. Once a dead pull hitter, Moustakas has rebounded from a season in which he hit just .212 with 15 homers and 54 RBIs.
"All that matters to him is winning," Royals pitcher Danny Duffy said, "but I think it would be extremely gratifying for him to get what he deserves, in my opinion."
The other AL final vote candidates are Red Sox shortstop Xander Bogaerts, Tigers outfielder Yoenis Cespedes, Yankees outfielder Brett Gardner and Twins second baseman Brian Dozier.
The 23-year-old Martinez, nicknamed "Tsunami," has been a big reason why St. Louis has the best record in baseball. The right-hander earned the fifth spot in the Cardinals rotation out of spring training and has gone 9-3 with a 2.70 ERA in his first full season as a starter.
The other NL final vote candidates are Rockies shortstop Troy Tulowitzki and pitchers Johnny Cueto of the Reds, Jeurys Familia of the Mets and Clayton Kershaw of the Dodgers.
"Whatever we got to do, I want to get them in there," Cardinals manager Mike Matheny said before facing the Cubs in Chicago. "That's the nature of the beast right now, the vote kind of rules. We're seeing some guys get some opportunities because of their fan support."
So is it OK to vote for the enemy?
"As long as they're voting for Cardinals," he said. "So absolutely. We're voting, too."
AP freelance writer Dave Royse in Chicago contributed to this report.