Delusion World — Gordon R. Dickson

There had to be a reason why that isolated human colony had been able to survive mankind's implacable enemies. But nobody had been able to get to the quaintly named Dunroamin to find out. If they had a secret defense, it could be the answer to a hundred planets' prayers. And Feliz Gebrod realized as he came in for a crash landing that he'd know the secret sooner than he'd expected. Except that what he encountered was a life-and-death riddle that had nothing to do with stellar defense. It was this: how can two mutually irreconcilable Utopias occupy the same space at the same time?