No, wait for a zero. Even if the test kit states a zero, there is still a trace amount remaining, which should sustain the A-bacs. Wait for the zero and give the N-bacs a chance to catch up.
It is good that the ammonia is dropping well, but each 1ppm NH3 processes into 2.7ppm NO2. So, the N-bacs colony will need to be about 3x as big as the A-bac colony in the end. That's part of the reason it takes longer for them to come around. (Also they don't start to build their colony until after the A-bacs start to process ammonia, so the A-bacs get a head start and the N-bacs need to grow a bigger colony. That's why phase two is usually the longest.