PHILOPENA--Henry Kitchell Webster--Bobbs Merrill ($2). Identical twin sisters are brought up separately, differently. Ten months after Celia's marriage, to which Cynthia could not go, she sends for Cynthia. Cynthia has no husband. Celia's husband is away. Will Cynthia please be Celia, just for 48 hours with no questions asked? It is very pressing. Very well, then. . . . Next day Cynthia (now Celia) clutches her newspaper. Celia (now Cynthia) is in a hospital, seriously smashed, unconscious. Ambiguous encounters, a detective, a furtive maid, mesh Cynthia-Celia in mystery. Apparently Celia-Cynthia is a criminal. Cynthia-Celia slowly finds out what kind, at the same time falling in love with the returned husband, who is puzzled but suspects no substitution, . . . Out of such stuff did Greek and Roman comedians fashion oldtime sidesplitters. Sometimes the situations were allowed to become broad. Author Webster, deft veteran, had ample ingenuity to twist his twins with the decorum and happy ending required of a first-rate American Magazine serial.