YESTERDAY MORNING, much to their surprise, the two men trying to outlast each other on a billboard in Allentown, Pa., found out that for every winner there need not be a loser.

On the 242nd day of a competition for a dream home that turned into a nightmare, Harold G. Fulmer III, the owner of radio station WSAN, which sponsored this crazy inertial war, announced on "Good Morning America" that both men would be awarded identical sets of prizes. To refresh your memory, the booty includes an $18,000 "Cozy Cottage" mobile home, a car, a new wardrobe and vacations in the Bahamas and Las Vegas.

Neither Mike MacKay nor Ron Kistler was able to see Fulmer make the declaration of largesse they had been hoping for. One of the rules in this war of wills was that TV sets weren't allowed in their tents, on a narrow ledge in front of the radio station's billboard. You think a radio station wants to promote television? The only clue the two got came when an ABC crew showed up to beam the men, via satellite, to New York, where Fulmer was sitting in a studio.

"It was a pleasant surprise," the 25-year-old Kistler said from the phone in his tent yesterday. "I'm looking foward to living in the trailer, driving the car, wearing the clothes and looking for a job, which I'll try to schedule around the vacations. I've gotten a little bored up here, but you get bored on the ground, too."

All this madness started in September, when WSAN announced the contest to promote its change in format, from rock to something a little more mellow: three people would be selected to sit each other out, and the triumphant winner would receive, courtesy of Love Homes, a mobile American dream. This came at a time and in a place that was being immortalized on the airwaves by Billy Joel as the very epicenter of unemployment, and the idea of sitting on a billboard to win a free home seemed more rewarding than what was available on the ground.

So these guys went after it with a vengeance. MacKay and his wife wrote out 47,000 entry blanks, Kistler 4,004. When their names were drawn from the tens of thousands of entries, you'd have thought they were just going to be handed that mobile home right then and there. But all they were really getting was . . .a chance to sit on a billboard!

Nonetheless, they were determined to win. Kistler said he'd stay up there until spring of '84. You just don't walk away from the American dream that nonchalantly. When MacKay announced, a couple of months back, that he was going to come down, everybody took notice. After all, hadn't this been the contest to end all contests, the one that was calling the attention of the world to the problems of unemployment in the land of steel and coal? And then it turned out that it was all an April Fool's joke.

In more ways than one: What had started out as a scheme to promote a radio station was beginning to turn into a bit of an embarrassment. People were calling up and writing letters accusing the station management of exploiting these men just to increase the station's ratings. Some of them wondered, how can you have guys up there for nine months without ever getting a chance to take a shower, eating food handed up by passers-by. Is this a human zoo, or what?

And so it was with a bit of relief yesterday that Fulmer said, "We felt that they had been up there an ample amount of time. We hoped it would end sooner. We thought about doing this much earlier, but then the police notified us that we were going to have a problem."

The problem was that in March a third contestant named Dalton Young was arrested for smoking dope up there on the billboard. After that, a few of the sponsors backed out.

"We've put it all in the hands of Gerry Gaffney, who put up the mobile home," Fulmer said yesterday. "He's taken over the promotion of this. There are some really big prizes in the works. I don't want to talk about them yet and mess it up. The film rights to this are in Gerry's hands now. We'll just see what happens. The boys will come down sometime in the next two weeks."

"I'm looking foward to getting reacquainted with my wife," said the 31-year-old MacKay. "She's only been allowed to visit me on Christmas, New Year's, Valentine's Day and Easter. I've learned the secret of life from this, which is: Don't enter any more contests."