Observations with double period integration (2Pobs) were used for the first time in 1972 to provide reliable detection of pulsars at low frequencies (Ustimenko 1983), where Pobs is the observed period. In 1973 observations of PSR 1919+21 were conducted with the period 4Pobs, during which interpulse emission (IPE) of that pulsar was found (Bruck and Ustimenko 1977b), later identified with observations at 38 MHz (Bash, Bozyan, and Torrance 1970, Bruck and Ustimenko 1977a). Later, because of the lower resolution, this method was not used, and its obvious advantages were forgotten. We returned to observations with 2Pobs integration period in 1987 to detect new pulsars, which proved necessary to overcome a number of difficulties and contradictions in identification of IPE.