SOCIALITE Phyllis Mack got a pretty good deal, as far as Park Avenue real estate goes. The philanthropist and wife of developer William Mack has paid a total of $20 million for two adjacent apartments sold by two different owners as a package deal, reports The Post’s Braden Keil. The asking price was $24 million. Former Treasury Secretary W. Michael Blumenthal had bought the smaller of the two apartments in the 14-room deal at 550 Park Ave. from Diana Vreeland’s estate and sold his portion for $5.4 million. The larger of the two units, owned by the Princeton-based Rita Allen Foundation, sold for $14.6 million.