MINNESOTA'S Twin Cities were separated at birth.St. Paul, the state's capital, has an "Eastern" feel to it. Summit Avenue, the city's tree-lined promenade, reflects the city's staid demeanor. Its Roman Catholic cathedral nearly dwarfs the domed capitol. And when St. Paul's blue-blooded women go to the opera, they wear wool overcoats. Ten miles to the west, Minneapolis's skyline glisters with glassy skyscrapers. This city, with numerous Lutheran churches, has a trendy, "Western" feel to it. On the west bank of the Mississippi River, its well-heeled women attend the symphony in full-length minks. But these dissimilar twins discovered each other this fall. As the Twin Cities baseball team, the Minnesota Twins, entered the American League playoffs against the Toronto Blue Jays, the cities became acquainted. After the Twins won and the World Series began last weekend against the Atlanta Braves, the Minnesota cities were friends. And now, with the World Series title within its grasp, the Twin Cities appear ready to "become family." Don't think a familial rapprochement has taken place, however. Minnesotans know the siblings soon will part and go their separate ways, even before the first ice forms on Lake Calhoun. People here recall a similar, brief friendship in 1987, when the Twins won the World Series. Still, the friendship - however short-lived - is wonderful to behold. In any given year, a love for walleye fishing and a dislike of the state's bitter winters are about all that Twin Cities largely Scandinavian and Irish populations can agree on. People in St. Paul - many of whom have not traveled to Minneapolis during the entire year - have reportedly been spotted haggling with ticket scalpers outside the Twins stadium, the Metrodome. A summer lottery determined the favored few who would see the series. But that hasn't deterred the rest of the residents on both sides of the Mississippi, most of whom have been glued to their televisions. There are World Series parties on nearly every street in town, and driveways and streets have become parking lots as neighbors descend on anyone with a big-screen television. Old-timers here tell of walking down tree-lined streets in the cities on warm summer nights during the 1930s and '40s when everyone's windows were open, and listening to sounds of Fibber McGee & Molly coming from each house. This week, however, walking by homes here in the evening, passersby - the five or six people in the cities not following the series - see frequent eruptions of mostly middle-aged men leaping into the air and "high-fiving" one another. Regularly scheduled meetings are being canceled, and streets in both cities have been jammed on game nights with cars and pedestrians scurrying to any restaurant or bar with a television. After each Series win, celebration has continued long into the night. The Twins World Series ride has united the cities unlike any other sporting extravaganza. The air-inflated, Astroturfed Metrodome - a skyless structure that most baseball fans agree has plasticized their sport - is even being praised by former critics on both sides of the river. The Twins in the past have fallen from grace with their fans as quickly as the temperature here goes from the 70s to the 20s each autumn. As did the Braves, the hometowns team finished the 1990 baseball season in last place, and at that time, few people from Minneapolis or St. Paul claimed the Twins as their own. As recently as last spring, Cleveland Indians, Boston Red Sox, and Detroit Tigers caps even seemed to outnumber Twins toppers. At least for now, though, St. Paulites and Minneapolitans are praising the Metrodome - and wearing Twins hats.