A local movie exhibitor, who was unpacking a 12-foot die-cut set of jaws in his office the other day, had a strange dream that night: a mammoth 30-foot-long great white shark leapt from the ocean and devoured John Travolta and Olivia Newton-John. With one chomp.

Of course, this dream has been two years, coming as Universal and Paramount prepared for the simultaneous release tomorrow of "Jaws 2" and "Grease," the two films most likely to compete for the summer's biggest box-office grosses.

And if the nation didn't have enough to choose from with the two movies, Americans can also spend the summer chewing on "Jaws 2" gum, hitting the discos in "Grease" jeans, sipping Coca-Cola from "Jaws 2" papercups while lying on "Jaws 2" beach towels, washing hair with "Grease" shampoo, downing "Jaws 2" cocktails at the local bar and listening to WCBN's "Greasemobile" playing the hits. It's the great American hype machine in full summer swing.

"Jaws 2," the continuation of the fish-bites-man story, opens at 628 theaters around the country, six in the Washington area: "Grease," the '50s rock 'n' roll musical starring Travolta and Newton-John, moves into 920 national theaters, one in the District.

By the time, the first projector starts flickering, the two studios will already have spent nearly $8 million in print and television ads to lure patrons to the box office. Not to mention marketing tie-ins worth an estimated $15 million.

"Jaws 2" beach blankets, etc.

Last week Universal began a teaser ad campaign, using the original "Jaws" shark fin logo with a tacked-on message: "Just when you thought it was safe to go back in the water . . ." The ads ran in newspapers and magazines and as a still image on TV. This week a bikini-clad water skier appeared before shark jaws in the print ads, and the 30-second TV spots included live-action footage.

The Paramount approach has been somewhat more restrained: On Sunday, ads proclaiming "'Grease' is the word" and displaying the slicked-back countenance of Saturday Night Fever" star Travolta appeared in newspapers.

Yesterday's newspapers brought Travolta and Newton-John together on the senior prom dance floor. Paramount also aired prime-time television spots this week, and offered local TV stations a one-hour TV special called "Grease Day U.S.A."

But while Universal has waged a full media blitz - including interviews with "special" still-photographer Susan Ford - as part of what "Jaws 2" producer David Brown calls "an open-house policy on the movie." Paramount has been holding its cards decidedly closer to its treasure chest. Some observers of the film industry speculate that the company thinks its great white shark competitor is a great white turkey.

"Just the opposite," says Paramount President Mike Eisner. "We screened the movie in Hawaii and it was a smash. Travolta is the biggest star in America today. If we thought we had a problem, we'd be selling our pants off."

Eisner admits that studio officials have been reluctant to discuss the film. Marketing director Gordon Weaver, publicity director Ed Kalish and producer Robert Stigwood all wouldn't talk about their film.

"We just don't want to oversell the product," Eisner says. "We simply want to let the product sell itself. 'Jaws,' 'Saturday Night Fever,' 'Star Wars' all sold themselves. That's what we're doing. We haven't found it necessary to give 'Grease' the kind of exploitation 'Jaws 2' is getting."

"We've taken an opposite approach," says Jaws producer Brown. "We decided early on that the movie was going to come out, and said, 'Okay, let's saturate.' The marketing of this film is a replay of the marketing of the first film by decree. Opening on the same day, in as many of the same theaters as possible, the same distribution overseas, even the creation of a book."

It was the phenomenal success of Peter Benchley's original novel "Jaws" - 9.75 million paperback copies in print - that helped pre-sell the original movie, which Brown estimates has been seen by 200 million viewers. So Universal hired adventure novelist Hank Searles to create a novelization of the "Jaws 2" script and sold the rights to Bantam Books, which published the novel on April 19. There are already 3.65 million copies in print, and the book has become the No. 1 best seller on the mass-market paperback lists.

"That was our best indication that there was a market for the movie," says Brown, "that the book became No. 1."

For Paramount, the pre-selling of "Grease" may well come from its soundtrack album, which has already produced the current No. 1 hit of John Travolta and Olivia Newton-John, "You're the One That I Want." The movie was co-produced by record business mogul Robert Stigwood, who also produced "Saturday Night Fever," the film with the $100-million soundtrack film. It's expected that several other hit singles will come from the "Grease" soundtrack, thus keeping the film in the ear of the potential viewer. The "Jaws 2" soundtrack, written by "Jaws"? "Star Wars"/"Close Encounters . . ." composer John Williams, has been timed to arrive in stores simultaneously with the picture's release.

Not to be outdone, Paramount licensed Pocket Books to create a "Grease" novelization. The publishing house hired its own Ron De Christoforo to pen the deed, having been pleased with his novelization of the Henry Winkler firm, "The One and Only." The paperback was released last month, and there are currently half a million copies in print.

"We were really hot for this one," says Pocket Books publicity director Mary Hall. "Ever since 'Star Wars' (a hugh success as a paperback), more and more novelizations are being done every day."

In addition to the book, there are "Grease" jeans and jackets from Sedgefield, shampoo from Helene Curtis, a Ford campaign featuring a car called "Greased Lightning" and a Pepsi-Cola promotion that will have 70 percent of the country's Pepsi trucks bearing "Grease" logos. In Baltimore, a radio station is roaming the streets with its own "Greasemobile."

"We've got our merchandising too'," says "Jaws 2" Publicist Al Ebner. "We've got T-shirts, iron-ons, beach towels, a "Jaws 2" game, glasses, a Wea & Ski tie-in, a Coca-Cola tie-in on drinking cups, candy, bubble gum. Heublein even has a "Jaws 2" drink made with Smirnoff vodka. We figure that we'll spend anywhere from $15-25 million to market this picture. But our biggest concern is getting the word out that this isn't just a tip-off sequel."

Producer Brown says that the merchandising can pay off royally in free advertising (every T-shirt is a free billboard, he points out), and can help make important decisions.

"Just two weeks ago we changed our ad campaign," he says. "We had planned to use the original 'Jaws' poster with the shark menacing the swimmer from below the water, but we found that the cover on the new novel, with the water skier and the shark out of the water, was outselling the old art. Bantam printed the book initially with both covers, and the new one outsold the old three-to-one. So we switched to the new one for the ads."

And so, D-day, or Shark-day, or Grease-day approaches. Collectors of trivia will want to note that there are 2,650 trailers for "Jaws 2" currently in theaters - the largest order for trailest ever made in Hollywood, according to Universal. And for the first time in its 102-year history, McCall's magazine has decided to feature a man alone on its cover: John Travolta.

But pity the poor exhibitor - the little man, faced with simultaneous openings, out there in Peoria, who months ago had to choose between "Jaws 2" and "Grease" for the summer feature at the Main Street Cinema.

Says Marvin Goldman, president of the National Association of Theater Owners:

"These are the kind of problems every exhibitor dreams of."