You could put together a pretty good team made up just of players who spent time with both the Cardinals and the Cubs.
Here's a fantasy roster of the all-time best players who wore St. Louis red and Chicago blue:
1B - Leon Durham -- Broke in with the Cardinals in 1980 and hit .271 with eight homers. The Cardinals offered Keith Hernandez in trade for Cy Young Award winning closer Bruce Sutter, but Chicago insisted on Durham instead. With the Cubs from 1981-88, Durham hit .279 with 138 homers and was twice named an All-Star. He returned to St. Louis briefly in 1989 and hit .056 before being released.
2B - Rogers Hornsby -- The Rajah played with St. Louis from 1915-26 and won six batting titles, an MVP award and led the Cardinals to their first World Series victory as player-manager. He ended up with the Cubs in 1929 after his hard head and salary demands caused him to be traded to and by the Giants and Braves. He won an MVP award in 1929, his first year as a North Sider but faded after that and was let go after 19 games with the Cubs in 1933. Hornsby caught on with the Cardinals where he hit .325 in 46 games before asking to be released to take an opportunity to be player-manager of the Browns.
3B - Frank Frisch -- Okay, so he was primarily a second baseman... But The Fordham Flash played 459 games at third base over the course of his career, so I think he can handle the job. Frisch hit .312 from 1927-37 with the Cardinals, won the MVP award in the World Series winning 1931 season and was a three-time all-star during a span the Cards won four pennants and two world championships. He also served part of his tour of duty as a player manager, leading the Redbirds to the 1934 World Series title as skipper. Frisch managed the Cubs from 1949-51, compiling a .418 winning percentage.
SS - Shawon Dunston -- He played shortstop for the Cubs from 1985-1995, often terrorizing first basemen with his rocket throws. He twice was named an All-Star. Dunston ended up with St. Louis in 1999-2000 where he played a utility role.
LF - Lou Brock -- The most infamous name in Cubs hot stove history, Brock hit .257 with 20 homers for the Wee Bears from 1961-64. He arrived in St. Louis in mid 1964 and hit .348 for the rest of the year to lead the Birds to a World Series win over the Yankees. Then Brock settled in for a Hall of Fame career in which he batted .297 for the Cardinals. He pilfered 888 of his National League record 938 stolen bases while with the Redbirds.
CF - Jim Edmonds -- Jimmy Ballgame was the most hated Cardinal of them all at Wrigley Field while he played with St. Louis from 2000-2008. He made flamboyant catches in the outfield, had a knack for throwing out baserunners at the plate and he hit .285 with 281 homers for the Cardinals. He played in two World Series with St. Louis and won eight Gold Gloves. After the Redbirds traded him to San Diego in 2009, Edmonds was released and ended up signing a free agent contract with Chicago -- largely to spite St. Louis and manager Tony La Russa. He hit .256 with 19 homers in half a season with the Cubs. But Chicago didn't offer him a contract in 2010.
RF -Max Flack -- No one ever bridged the Cardinals-Cubs rivalry better than Flack -- and his counterpart Cliff Heathcote. The players were swapped between games of a doubleheader at Wrigley Field on May 30, 1922. Flack, A Belleville native, played for the Cubs from 1916-22 and hit .277 with 23 homers. From 1922-25 hit .278 with seven homers for the Redbirds.
SP - Dizzy Dean -- A Cardinal in 1930 and from 1931-37, Ol' Diz Won 30 games for the Redbirds in 1934. He was National League MVP once and finished second twice. But Dean hurt his arm in the wake on an incident in the 1937 All-Star Game when a line drive broke his toe, altering his pitching motion. Dean was sold to the Cubs in 1938, mostly as a gate attraction. From 1938-41 with Chicago Dean only won 16 of his career total of 150 wins.
SP - Rick Sutcliffe -- A Cubs starter from 1985-91, Sutcliffe was the runner up for the Cy Young Award and won a pair of All-Star Game berths. He was 82-65 with a 3.74 ERA for Chicago. In 1994, the 38-year-old version of Sutcliffe signed with the Redbirds and went 6-4 -- with a 6.52 ERA -- in 16 games before calling it a carer.
SP - Ernie Broglio -- The salt in the Cubs' Lou Brock wound, Broglio was 70-55 with a 3.74 for the Redbirds from 1959-64. He won 21 games in 1960 and had 18 wins in 1963 to entice the Wee Bears to make a deal for him. But Broglio was 7-19 with a 5.40 ERA for the cubs from 1964-66 before hanging up the spikes at 30 years old.
RP - Lee Smith -- A Cub from 1980-87, Smith saved 180 games with a 2.92 ERA in the cozy conditions at Wrigley Field. He came to the Cardinals via the Red Sox in 1990 and remained until 1993. He saved 160 games for St. Louis with a 2.90 ERA, making him a rare example of a player who was fairly equally successful for both clubs.
RP- Dennis Eckersley -- He played for the Cubs in 1985 and 1986 and was a mediocre starter. Shipped off to Oakland, Athletics manager Tony La Russa made Eckersley a reliever -- and he became one of the best in the game at his craft. In 1990 with Oakland Eck pitched in 63 games and had an 0.63 ERA with 48 saves. He was near the end of the line when he was picked up by St. Louis for the 1996 and 1997 seasons. But Eckersley was able to save 66 games despite a 1-11 record and a 3.58 ERA.
RP - Bruce Sutter -- Available to the Cardinals because of the emergency of Smith, Sutter was a four-time all-star and a Cy Young Award winner during his 1976-80 stint with the Wee Bears. Sutter played for the Redbirds from 1981-84 and led the league in saves twice compared to three times with Chicago. But St. Louis made up for the difference by winning the 1982 World Series with Sutter racking up saves two innings or more at a time.