The 56-story First International tower in Dallas, one of that city's biggest office structures, is best known as the "Ewing Oil" building for its role in the "Dallas" television show, but it also has another modest claim to distinction.

It is one of a dozen or so buildings in the United States and Canada to be equipped with double-deck elevators--elevators that carry two attached cabs in a single shaft, increasing the carrying capacity of the elevator system and leaving more floor area for use as rentable space.

Citicorp Tower in New York has double-deck elevators. So do the Time-Life Building and the Standard Oil of Indiana building in Chicago, the John Hancock Tower in Boston, the Bank of Montreal building in Toronto and a few other giant office structures in major cities.

Almost all of them were built and installed by the Otis Elevator division of United Technologies Corp., which says that "no elevator system that exists today can empty a large, heavily populated office building more quickly."

According to Otis, a hypothetical office building of 38 stories, with 200 persons working on each floor, would require 26 conventional elevators in four banks, occupying 83,419 square feet of "core area," but the same capacity could be created by 18 double-deck elevators in three banks, taking up only 56,192 square feet.

"We figure you can use two-thirds to three-quarters the number of elevators you would need for conventional operation," said Otis engineer Robert Brown. "We have used the calculation on relative cost that a double-deck system would cost 125 percent of what a single deck system would cost," with the increased rentable space more than compensating for the difference.

Despite these presumed advantages, double-decking of elevators is not exactly sweeping the country. The first ones were installed in New York at 60 Wall St. in 1934. Real Estate Magazine said they "stabilize service in rush hours, to the great satisfaction of tenants and visitors, score net saving in cost of operation, bring new rentable area into profitable use and can run single deck on dull days."

Half a century later, however, there are still no more than 20 office buildings in North America that use double-deck elevators, which Otis has only recently begun to promote in earnest. There are several reasons why they never caught on, according to engineers and architects.

For one thing, they require a two-level lobby, so that passengers in both the upper and lower cabs can exit after descent. Also, they are especially suited to large buildings where the volume and flow of traffic lend themselves to the use of two-level cabs. According to a report in Consulting Engineer magazine, "the need becomes most acute in a high-rise building occupied largely by one organization, where people start work and leave for home at about the same time and create exceptionally heavy traffic peaks for elevators to handle."

Another problem, Otis officials acknowledge, is consumer resistance. Architects and tenants often dislike them because they foresee problems in traveling a single floor up or down, which may require the installation of supplemental single-cab elevators. Many persons are uncomfortable when the elevator in which they are riding stops but the door doesn't open because only the other cab is taking on or discharging passengers.

Peter Thompson, an Otis spokesman, said "the idea has had limited acceptance. People don't want to go down to the first floor to catch the elevator, they want to know, if I work on the 15th floor, and I want to see my girlfriend on the 14th, do I have to go all the way down?"

According to Brown, that problem can be solved by programing the elevators so that if there are calls for traffic between floors, an elevator can leave the lobby empty and travel "unrestricted" from floor to floor above.