Complex sentence: The penthouse on the top 2 floors of the structure which totaled 20,000 square feet (1,858 m²) was priced at $58 million, however, after failing to sell for years, it was separated into four different units.
Split into simpler sentences: The penthouse on the top 2 floors of the structure which totaled 20,000 square feet (1,858 m²) was priced at $58 million. This apartment, after failing to sell for years, was separated into four different units.