Which of the original escs do you run, two BL 20 SD or the BL 30 2 in 1?
If it is two BL 20 you may use a V-lead connected to just one channel on the RX. Perhaps it is necessary to calibrat each esc anew. The motors should start at the same time.
If you have connected the escs to different channels on the RX (e. g. 4 and 7) it might be that there occurs a time lag based on the signaltimeframe. It depends on your RC-equiment. Modern ones, especially 2,4 GHz and in fast response should not really show this time lag effect - then it should be almost reall time. You hardly can do anything against it on the TX. Use a V-lead and the lag is a problem of the escs or the motors.