LOS ANGELES, CA.- Call it the Irony Curtain. The concrete wall that divided the city of Berlin for 28 years is suddenly splitting a segment of Los Angeles' art community just as the 20th anniversary of the wall's falling nears. The trouble began earlier this month when the Wende Museum installed several segments of the original Berlin Wall on Wilshire Boulevard. Kent Twitchell, whose larger-than-life paintings cover entire walls and sides of freeways, said he planned to "bookend" two sections of the wall with portraits of President John F. Kennedy, who denounced the barrier in a Berlin speech in 1961, and President Ronald Reagan, who famously demanded, "Tear down this wall!" shortly before it came down in 1989. But as he rushed to finish the portraits, Twitchell said he was told by organizers that he could leave one of the paintings in his studio: There was no room for both. "They said there would only be room for one and they just assumed it would be Kennedy," the disappoi