‘Back to the Future’--Filming in Double Time : Movies: Two sequels to the megahit that took in $358 million at the box office are coming to the present. The first opens this week; the second is scheduled for summer.

“Synchronize your watches,” proclaims the massive billboard hovering over the Sunset Strip, keeping a vigil on the number of hours, minutes and seconds remaining until the release of “Back to the Future Part II,” a follow-up to the top-grossing film of 1985.

Similar billboards have been counting down in Universal City and in New York City’s Times Square. There is no name identification. Just the familiar image of Michael J. Fox, who, as the adolescent time traveler Marty McFly, helped sell $358 million worth of tickets worldwide for Universal Pictures.

A payoff like that begs for a sequel, and now, after a 4-year wait, Universal has not one but two sequels, shot back to back and scheduled for release within six months of each other.

“Back to the Future Part II,” which takes Marty, his girlfriend Jennifer (Elisabeth Shue) and wild-eyed Doc Brown (Christopher Lloyd) to the year 2015, opens Wednesday; “Back to the Future Part III,” set mostly in the Old West, is set to surface at the beginning of next summer. The studio has yet to determine plans for the release of the sequels’ videos.

The price tag for the two is about $80 million, which director Robert Zemeckis acknowledges is $10 million to $15 million less than if they’d been shot separately, but still more than twice the industry average.

“Doing both at the same time was quite a risk,” said Tom Pollock, president of Universal Pictures. “It was much more expensive initially . . . like laying out the money for four films at once.”

Zemeckis and co-producer/screenwriter Bob Gale, faced with a “floodgate of ideas,” pitched the two-sequel idea to the studio last fall.

“It seemed an audacious, insane proposal on the face of it,” said Gale, “but it made sense economically and creatively. No sets had to be reassembled, no schedules coordinated three years down the line. And since Michael had to play a high school kid, waiting the few years it usually takes for a studio to mount a big special-effects movie would have been pushing the outside of the envelope.”

“Part II” begins where the original left off. “It’s your kids, Marty. Something’s gotta be done about your kids,” warned Doc Brown--and Marty and Jennifer, zooming off in the DeLorean time capsule, take his words to heart. What they encounter is neither Orwellian nor Lucas-like, but rather a humorously depressing picture of their middle-aged selves . . . and, later on, the specter of an “alternate 1985” in which greed and environmental neglect have run amok.

“We decided there was no way we could predict the future accurately,” Gale noted, “so we decided to have some fun with it.”

What surfaced was a true modern-day fantasy, a world in which the Cubs win the pennant and the justice system (devoid of lawyers) works well. Sight gags and inside jokes abound. “Surf Vietnam,” reads one travel poster. Playing at the local theater is “ ‘Jaws 19,’ directed by Max Spielberg.”

A trailer with highlights of “Part III” is tacked onto the end. “Depending on what happens with this one, it’s either the world’s most brilliant marketing ploy or the most audacious affront,” said Fox, talking by phone from the Northern California set where “Part III” is still in production. “It could be perceived as presumptuous, but the last thing Bob Zemeckis wants is for people to think he phoned this one in. He’s a great ‘You ain’t seen nothing yet’ kind of guy.”

The two Bobs have collaborated ever since their days at USC Film School, sharing screenplay credit on three Zemeckis films--”Used Cars,” “I Wanna Hold Your Hand” and “Back to the Future”--as well as on Steven Spielberg’s “1941.” “Bob G. is the thinker, the strategist,” Fox explained. “Bob Z. is the doer, the one who takes the seeds that have been planted and reads even more into them than originally intended.”

Zemeckis said that doing a sequel was the furthest thing from his mind when he and Gale knocked out the first “Back to the Future” draft in 1981. The project, originally developed at Columbia, had been turned down by every major studio, and prospects for getting it to the big screen were dim.

“It was before ‘E.T.,’ ” said Gale, “and no one was sure there was a market for a ‘sweet’ film. Everyone told us to take it to Disney, which, in the pre-Eisner-and-Katzenberg days, wasn’t up for a film about a romance between this kid and his mother.”

A month after his “Romancing the Stone” took off at the box office in 1984, Zemeckis’ time-warp comedy got the green light from Universal. When the studio asked him to do the sequel, however, the director had second thoughts.

“I didn’t know if I wanted to re-enter Back-to-the-Futureland,” said Zemeckis, from the Jamestown, Calif., location. “But it became clear that the train was going to leave the station whether I was on it or not. Better to have some control and be faithful to the original, I figured, than to turn my back and walk away.”

But the train wouldn’t go anywhere, he insisted, until he squeezed in at least one other project. That project turned out to be “Who Framed Roger Rabbit,” the No. 1 film of 1988.

There was no master plan or grand design for the trilogy, he said, just a determination not to clone the original.

“We saw this as a saga, along the lines of ‘Godfather II’ and the ‘Star Wars’ films,” Zemeckis said. “The same characters, the same town seen through different times. The thought of going back into the first movie and shooting the same scenes from a different perspective excited us. My challenge as a film maker was to redefine this thing they call sequel .”

Zemeckis calls “Back to the Future II” his toughest shoot yet. The script calls for the actors to play several characters in the same shot, necessitating elaborate and precise camerawork. (In one scene, Fox plays three roles at once: Marty at 47 and his teen-age children--one boy and one remarkably attractive girl).

Shooting one film while in post-production on another also doubled the director’s workload. In between shots of “Part III,” he’d head for his mobile editing room to work on “Part II.” During a recent stretch, Zemeckis shot one movie from 6 a.m. until sundown, then caught a plane to Burbank, where he did post-production work on the other until late into the night. He then checked into the Sheraton Universal, got up at 5 a.m and took the 50-minute flight back north.

Though admitting to fatigue, he downplayed the commute: “Only 10 minutes longer than it takes me to get from Santa Monica to Burbank at rush hour. And I got to sleep on the plane.”

Fox too had his hands full. For six weeks, while filming of “Future” II overlapped the taping of the final season of TV’s “Family Ties,” he worked days at Paramount playing Alex Keaton and nights (until 2 a.m.) at Universal playing Marty McFly. The birth of his son, Sam, in May compounded the intensity.

“I’m not the martyr I was five years ago when I juggled the two,” he said. “Though I’m not known for throwing my weight around, when I’d get exhausted, I’d say, ‘Go fire me.’ If I’d tried that last time, I’d have lost both jobs. . . . The horrible thing is that we’ll still be shooting when this one opens. If it bombs, I’ll be there in Monument Valley, Utah, putting my hand to my head . . . ‘Oh, my God!’ ”

Industry analysts, for the most part, don’t think he has much to worry about. “The film has a built-in audience, the concept is easily transferable to a sequel, and some of the competition has been pushed back,” said Jeffrey Logsdon, an analyst with Crowell, Weedon & Co. “People are keyed up and ready to go. It’s the most anticipated film of the Christmas season.”

Whatever the outcome, say Zemeckis and Universal executives, there will be no “Back to the Future Part IV.” Entertainment lawyer Peter Dekom of Bloom, Dekom & Hergott, has heard that one before. “They say they’ll close with this script . . . but that’s only until they need revenues. They know it and so does everyone else.”