NOT THAT THIS IS a Guinness World Record, but this is probably the first time that a publisher is going on a tour for a book he didn't write himself. And it's the first time that Two publishers are going on one tour. Robert L. Bernstein, president of Random House, and James Mitchell, chairman of Mitchell Beazley, are going on tour for the Random House Encyclopedia, their joint venture. The duo are going to do the entire route . . . talk shows, radio, television, luncheons, probably even autographs . . . for two solid weeks coast to coast with a day off for Rosh Hashana. Mitchell is the editor in chief of the Encyclopedia which was nine years in the making, and, with publication set for September 30, has 175,000 copies in print. It's a grueling tour, and a two-sided one, with Bob Bernstein as publisher and James Mitchell as editor. The investment in the book is huge with the ad campaign already budgeted at over a million dollars. So Random House has got to do a hard sell on an encyclopedia that enters an already highly competitive field, and one in which door-to-door rather than bookstore selling has been the way of life up till now. It will be interesting to get Mr. Bernstein's impression of the way the author half lives when he gets back from his tour. He'll be looking at life from both sides after this.