SOMEWHERE IN MIDDLE America, or so I read in a magazine, there are two women who write romance novels under a single pseudonym, cackling and falling off the couch with glee at each irresistible idea. Meanwhile, somewhere in middle Canada, David Parry and Patrick Withrow appear to have had a marvellous time concocting The Jacamar Nest (Macmillan, 369 pages, $19.95 cloth) while tossing back a few friendly brews. I could be wrong, of course, but this theory does explain why The ]acamar Nest opens so buoyantly and consistently stays up there. Writing together is something Parry and Withrow evidently enjoy doing; happily, they`ve succeeded in giving the reader a good time as well. Their American hero, Harry Bracken, is a 40-year-old insurance investigator whose 17 years of international secret service came to an abrupt end when he killed the wrong man - deliberately - because someone else had killed the wrong woman - his wife. Back in the United States, Harry`s talents are being grossly underemployed until he runs across the terrorist activities of a Palestinian organization that has figured out an effective way to raise money for the cause: corporations pay big bucks to protect their expensive images when threatened with, for example, poison in their beer or sticky accelerators on their cars. Harry comes through, of course, with the help of the obligatory beautiful but liberated girl and an eccentric mentor who`s like a smart Alec Guinness playing Obe Wan Kenobe in a motorized wheelchair. Good, not-so-clean fun.