SAN Diego and the government of Mexico began talks earlier this month on a new $2 billion airport to straddle the United States-Mexico border."San Diego is in critical need of a new airport [and] Tijuana's growth is also constrained by its limited capacity," says John St. John, a consultant working with the San Diego Association of Governments. "A North American free-trade agreement is going to stimulate tremendous demand for air services which neither of the two current airports can meet," Billed as the first truly "international" airport, the San Diego proposal is to build a 12,000-foot runway adjacent to the border with a taxiway connected to the existing runway at Tijuana airport. A single control tower would direct planes to use either the Mexican or US runway. A cooperatively run tower would be necessary because aircraft landing on either runway would use the airspace of both countries. Mexico's Secretariat of Communication and Transportation has an alternate plan - putting most improvements on Mexico's side of the border. Those plans call for another, parallel runway connected by taxiway, tunnel, or both to terminals in the US. San Diego proponents argue the Mexican plan is technically flawed. There is no room for two parallel runways and the proposed length, 10,500 feet, is not enough for a fully-loaded Boeing 747 to take off for Asia, they say. There are also problems with an instrument approach, they say. By opting for a version of the US plan, Mr. St. John says, "Mexico could conceivably spend virtually nothing on this and yet reap tremendous benefits." Meanwhile, there is local opposition to the San Diego plan, mostly from residents near the site in the Otay Mesa area. Opponents also say it would be too far from downtown San Diego. A green light to study the concept came from Mexican President Carlos Salinas de Gortari after a visit by San Diego Mayor Maureen O'Connor late last month. But the Salinas administration is divided over the airport plans, a source close to the project says. Further bilateral talks are scheduled for mid-July.