Following Wall Street's captivation with whether the nation's uninsured would take to public exchanges to sign up for coverage under the Affordable Care Act, investors may be missing another wave of Americans signing up to a similar vehicle in the private sector.

They're called private exchanges and employers and insurance companies are aggressively moving employees, their dependents and retirees to them in droves. It's a trend that benefits the stocks of consulting firms and health insurance companies.

Under the health law, the uninsured go to the federal or state marketplaces known as exchanges with help buying coverage from a government subsidy. Under the private exchanges, workers and retirees go to a similar web site or exchange, often with a credit to buy private coverage.

The move to private exchanges is becoming a boon to employee benefits consultancies Aon Hewitt (NYSE:AON), Mercer, a subsidiary of Marsh & McLennan Companies (NYSE:MMC), and Towers Watson (UNKNOWN:TW.DL). The companies have yet to specifically disclose revenue or profits related to these businesses, but the sales are growing if the client base is any indication.

At Aon Hewitt, for example, the growth in employees and employers signing up to the company's exchange has soared. In the first year of the Aon Hewitt Health Exchange, there were three large employers and 150,000 "covered lives" in 2013.

This year, there are more than 600,000 covered lives from 18 large employers, Aon Hewitt said. Participating employers include Sears Holdings, Darden Restaurants, and Walgreen.

"Private exchanges are rapidly becoming a core health benefit strategy for many employers, allowing them to maintain their commitment to health benefits while creating a more effective and efficient health care ecosystem," said Cary Grace, CEO of Aon Exchange Solutions. "Our data shows 33% of employers are considering a private exchange strategy for active employees within the next 3 to 5 years and many more have already adopted this strategy for retirees. At Aon, we are seeing steady interest in our suite of exchange solutions from employers of all sizes and industries, and we expect that interest to continue."

Meanwhile, Mercer which launched its exchange last year for enrollment at the beginning of this year said it has 67 employer clients "with 289,000 (covered) lives across employees, retirees & family members," Mercer spokeswoman Stephanie Poe said.

Mercer said the interest in buying coverage from exchanges ranges from small to large employers. "Clients range from 100 employees to 33,000, across more than 15 industries," Poe said. Mercer's client list includes Petco and Kinder Morgan.

Giant health insurance companies, too, want to form exchanges and are in the early stages of exchange development. Cigna, Aetna, and several Blue Cross and Blue Shield plans are among those adding members through private exchanges.

On Aetna's first quarter earnings call, CEO Mark Bertolini said private exchanges helped the Hartford based insurance giant add nearly 300,000 members in the company's "large group commercial business" in the first quarter.

"These gains were attributable to new customer wins in public and labor, continued growth in our network access products and new, private exchange membership," Bertolini said.

Aetna and other health insurance companies say they foresee an even bigger boom as employers this summer evaluate their benefits options for 2015. Those discussions are under way in the weeks and months before fall open enrollment, the annual ritual when workers can change their benefit plans.

"We have a number of proprietary private exchange opportunities we're pursuing as a next evolution of the private exchange model, and we think that's going to have more economics associated with it," Bertolini told analysts and investors on the company's April 24 first quarter earnings call. "So we think this trend will continue and we hope that if we can prove that we're able to manage trend more effectively over time, that employers will consider moving to defined contribution sooner – and as a result we'll create more of an opportunity in the insured marketplace."

How big of a difference will private exchanges make in these companies' bottom lines? For now, it's hard to say for sure. But the potential for private exchanges is enormous -- according to Rich Birhanzel of Accenture's health administration services division, potentially as many as 40 million Americans might be enrolled in them by 2018. It's a story investors will definitely want to watch closely in tandem with the better-publicized Affordable Care Act.