AN AFFLUENT COMBO of monied Boomers and six-figured Gen Xers have joined their parents in turning up the heat on the second-home market. ¶ "I just put two couples into their second homes in Hawaii," says Gregg Fujita, a broker in the San Francisco Bay Area. "They're selling their big family homes now that their kids are all out, taking their equity, and buying second homes. And they're buying smaller homes - townhomes - in the Bay Area. ¶ This is one new market that Fujita has seen cranking up in recent months, particularly after an astonishing run-up in home values in the region finally plateaued. And it's not just a Bay Area bubble. Based on a back-of-the-envelope calculation of industry statistics and figures from the U.S. Census Bureau, an economist for the National Association of Realtors (NAR) estimates that there are some 6.63 million second homes in America. And by all accounts, the figure is growing by income leaps and construction bounds. ¶ And while the youngsters are in on the action, it's the Baby Boomers who are leading the two-home trend. According to NAR's latest survey on the second-home market, the average second-home buyer is 47, married, has a family income of $86,000, and tends to use the alter-address for recreation. In some cases, they're counting on the fruits of a lifetime of professional labor - often by the husband and the wife - to buy the home. In others, it may be an inheritance being put to use to fulfill a lifelong dream. It's not at all uncommon for the kids to be out of the nest and off the family payroll. And in virtually all cases, the buyers are pretty well-heeled.