LESLIE HILL This journey brings together two artists of the same gender, age, weight, height, coloring, hair style and shoe size, who happen to possess nearly identical wardrobes and whose own parents are not unknown to mistake them for each other.   There's one big difference: Helen grew up in Britain, dreaming of America; Leslie grew up in America dreaming of Britain. For Helen, the journey is a wish fulfilled; for Leslie it is a return home after ten years in the UK. HELEN PARIS