Pan American World Airways revived airline discount coupons yesterday, announcing a plan that potentially gives travelers the biggest bargain fares ever and could trigger a new underground market in airline coupons.

By taking a $99 National Airlines flight to Florida, a traveler can get a coupon to buy two tickets for the price of one on Pan Am trips as long as the $1,938 flight from New York to Australia.

The promotion is the first effort to use National to build business for Pan Am since the two airlines merged earlier this year.

The "two-for" coupons will be issued on National flights from Washington or New York to Florida from now through April 9 and are good on selected Pan Am routes.

The "two-for" coupons will be good on Pan Am flights from New York to Los Angeles or San Francisco and from the United States to Australia, Fiji, Singapore, Thailand, Kenya, Costa Rica, Guatemala or Germany.

The free second ticket can only be used by a relative of the person who buys the first ticket, and the two must travel together. The two-for one tickets must be used between May 15 and Dec. 15 and are not valid during certain high-travel periods.

Even with the restrictions, however, the Pan Am "two-fors" offer travelers the biggest bargain ever available on an airline discount coupon plan.

The free second ticket is worth almost $600 on a New York to California round trip at regular economy fare $912 on a round trip to Frankfurt, Germany and $1,938 on the trip halfway 'round the world from New York to Sydney, Australia, and back.

The Pan Am offer could lead to the same kind of black market in coupons that sprang up after United Airlines and American Airlines issued halffare discount coupons last year. United started giving the coupons to boost traffic after a long strike and American matched the offer to keep from losing business.

A flourishing market developed in which the coupons sold for $30 to $75 each, enabling travelers to save half the price of a ticket on a domestic flight costing up to about $600.

Pan-Am officials said the coupon plan was not intended to help National compete with discount air fares on the Washington/New York to Florida run, but to tie together the National and Pan-Am routes.

The "two-for" tickets cannot be cashed in for a refund or traded for other tickets. They will not be honored for transatlantic travel leaving the U.S. during July or returning to the U.S. in August or going to the Far East via Hong Kong in July or returning from there during August.