When two anti-Castro activists were arrested on weapons charges in Miami, federal prosecutors filed the indictment in Fort Lauderdale -- seen as an ''insult'' by the pair's supporters. The legal team for Santiago Alvarez and Osvaldo Mitat tried in vain to move the case back to Miami, arguing it was the only way the Cuban exiles could get a fair jury trial. As the Sept. 12 trial approaches, their attorneys have come up with a new tactic: Allow Miami-Dade residents to sit alongside Broward residents in the jury pool so that some Cuban Americans might be selected. It's legal, but it may be a long shot. And it points up the sensitive issue of choosing jurors in the bordering counties for federal trials in which race or ethnicity can make the difference between a verdict of guilt or innocence.