Greystar Real Estate Partners is on the verge of acquiring Riverstone Residential, sources have confirmed. The combined company would have a massive portfolio of more than 391,000 units. A formal announcement is expected Wednesday.

The marriage would pair the two largest companies in the cutthroat apartment management business. Charleston, S.C.-based Greystar, the No. 1 apartment manager in the 2014 Top 50, oversees more than 214,696 units. Dallas-based Riverstone, the No. 2 manager, operates 176,319 units. Former Riverstone CEO Walt Smith departed in September 2013, replaced by the company's co-founder Terry Danner.

Late last year, Greystar, piloted by longtime CEO Bob Faith, acquired more than 4,300 units from Inland American Real Estate Trust Inc. Earlier in the year, it struck a $1.5-billion, 8,010-unit deal with Equity Residential. Riverstone built its portfolio through a number of acquisitions, including 23,000 units from Stratus Real Estate in May 2007 and 21,000 units from RMI in 2008.

The deal gives Greystar a suddenly much larger portfolio in which to spread overhead costs, achieving greater economies of scale in the dog-eat-dog property management business. Over the past decade, property managers have been forced to increase investments in such areas as property management software and digital marketing. Additionally, the massive company would give Greystar even more clout to court much-coveted institutional clients.

Neither Riverstone nor Greystar returned calls for comment.

For an exclusive look at how Riverstone's and Greystar's rivals view the merger, click here.