The nation's airlines, anxious to fill empty seats, are offering free companion tickets to travelers who book flights by Friday and holding their breath to see if it increases revenue.

With the industry already suffering from the high cost of jet fuel and a slowdown in travel, the airlines can ill afford the additional losses that might result from such steep discounts.

So why are they handing out what amounts to a 50 percent fare cut on already discounted advance purchase fares?

What it amounts to is a gamble that the two-for-one gimmick will attract travelers who otherwise would have stayed home. But according to travel industry officials, the idea may be backfiring: Many of those buying the "twofers" are passengers who are rebooking flights rather than making new travel plans.

"These kinds of promotions in my experience have never been particularly big winners," said Bruce R. Nobles, former head of the Pan Am and Trump shuttles.

"I don't think the airlines are going to make any money on this," said Dan Bohan of Omega World Travel, one of the Washington area's largest travel agencies. Bohan said that his agency had booked thousands of the two-for-one tickets, but that most of them had been reissues of old tickets.

In some cases, according to Bohan, the cost of issuing the tickets has been greater than the commission generated, making the fare promotion costly for the travel agent as well.

Unlike many discounts, the two-for-one fares were started by the strongest players in the industry and could do the most damage to weaker carriers that can ill-afford such price slashing. When first offered in the fall by United and then American, travelers in limited markets were offered an identical ticket for a companion for $50.

The gimmick quickly spread through the industry and into virtually every market. Then last week, Northwest Airlines dropped the charge for the companion ticket, and other major airlines followed suit.

"One does it, and then another does it, and then another betters it until they're all losing money," said Bohan. "If I were the traveler, I'd go get it, because it's the best deal out there."

Airline representatives said that the gimmick has produced a big increase in bookings during what is traditionally a slow time for the industry, but most of them said it is too early to tell whether the idea is a moneymaker or not. "It really is difficult to forecast whether it's going to be positive or negative economically," said Al Becker of American Airlines.

Northwest Airlines spokesman Doug Miller said that bookings were up on Nov. 19, the first day of the promotion, by 23 percent over the same day a week earlier and up by 24 percent the following day. Although he couldn't say what percentage of that increase might be rebooking, he said that the airline's marketing department believes most of it was new business.

The two-for-one offer is not available for travel during the holiday period and is also unavailable in most cases for travel to such popular winter destinations as Florida.

Even if the fare promotion fails, First Boston Corp. analyst Paul Karos in New York said there may be a less tangible benefit. In a weakening economy where travel is often cut back to conserve cash, "it kind of keeps the consumer interested in flying," he said.