The sprawling penthouse that tops 212 Fifth Avenue almost wasn’t: The developers planned to split the condo unit and market it as two separate homes. Now, the latest offering plans for the office-building-turned-residential condominium show that developers Madison Equities and Thor Equities decided to combine the two units into a 10,079-square-foot triplex. The decision comes as other property owners do the exact opposite — separate large condo units to match the softening luxury market.