Just as it wobbles along a precarious ledge between solid drama and airborne comedy, "The Skeleton Twins" maintains a tremulous middle distance between success and failure. Its stars, Bill Hader and Kristen Wiig, always two bright spots on the increasingly dismal "Saturday Night Live," do stellar work as Milo and Maggie Dean, twins slogging through early adulthood burdened with a busload of baggage.