New pulsation modes in two delta Scuti stars of Praesepe cluster, BQ and BW Cnc, have been detected during the STEPHI VI campaign in 1995. In particular, 3 frequencies for BQ Cnc and 9 frequencies for BW Cnc have been found above a 99% confidence level. The possibility of the presence of a g-mode present in BQ Cnc is discussed, considering its binarity. The effect of mutual interference between very close detected frequencies in BW Cnc during the observations, is also considered. This last effect reveals the necessity of long-period observing runs, in order to avoid its influence in the final number of detected modes. In such situation, studies of secular amplitude changes can be strongly affected.