The home real estate market may be in the doldrums, but one segment of it appears to be on the verge of a a boom: equity sharing.

Six months ago, barely a handful of firms nationwide were involved in joining home buyers with investors. The idea that many first-time and moderate-income purchasers would want to own a house or condominium with a nonresident investor-partner stuck most industry leaders as poppycock.

The overwhelming majority of home buyers, they said, will wait for interest rates to come down, or look for seller financing, rather than giving away any of their rights of ownership to strangers.

Equity sharing, in the words of St. Louis real estate executive Les Glickman, "will be a flash in the pan. It'll never be attractive to more than 1 or 2 percent of the buyers in the market."

But that conventional wisdom is being proved wrong. Buyers, investors and brokers report huge gains in usage of the equity-sharing approach. Real estate firms in dozens of cities--including one 3,500-office brokerage chain--are jumping headlong into equity sharing.

Individual entrepreneurs in Texas, Virginia and California now talk confidently about capturing 25 percent of the new and resale housing market within three years via equity sharing.

Large national franchise organizations are at work on their own variations of the concept, the first of which has just been unveiled by Control Data Corp.'s Electronic Realty Associates (ERA).

Dubbed the "partnership mortgage," the plan pairs cash-short home buyers with small-scale investors. Prospective buyers anywhere in the country receive computer-designed programs matching their financal situations to potential shared-equity home purchase options with private investors. Commonly, no down payment is required from the home buyer whatsoever.

The investors--typically high-tax-bracket professionals known to local ERA franchisees--receive tax-shelter benefits plus a large chunk of the long-term capital appreciation of the property. They provide the downpayment and some portion of the monthly principal and interest costs.

The "partnership mortgage" plan allows the homeowner-occupant the right to sell the house or buy out the investors' capital interest at any time. ERA national president Michael Jackson of Shawnee Mission, Kan., estimates that, during the program's January and February startup months alone, the firm has initiated $12 million worth of partnership mortgage in over 300 transactions.

More typical of the activity going on in large metropolitan areas are Houston's "SAVE" (Shared Appreciation Venture Equity) program and Washington's "The Venture Group."

The Venture Group, begun by former Carter White House staffer Veronica Pickman, specializes in condominium equity sharing--especially in conversion projects.

The firm often seeks to assist tenants in rental buildings being converted to condominium ownership to remain in their units as co-owners.

By locating outside investors who are willing to split or pay all the downpayment costs of the converted condo, as well as subsidize part of the monthly mortgage payments, Pickman's company fights the "displacement" trend. Elderly, fixed income tenants in particular benefit from the co-ownership approach. The firm earns sales commissions when each joint-venture deal goes through.

Pickman' asociate, Agnes Davis, says their concept "makes economic sense out of the tough conditions many people face in this high-rate economy." It takes the Reagan economic message literally--solve your problems with private enterprise and the tax system--and achieves some mutually beneficial objectives for consumers at the upper and lower ends of the income ladder.

The 1981 tax law depreciation changes made highly leveraged small investments in real estate "fantastic deals for people in high tax brackets," Davis said. The tax benefits can be so significant, she insists, that the properties financed needn't appreciate in value by more than 3 or 4 percent a year for the investor to do well on joint ownership.

For the moderate-income tenant of the converted unit, on the other hand, said Davis, "the benefits of stable occupancy and affordable monthly costs are more than worth the share in possible future profits given up to an investor.