Part A is the buffer and part B is the calcium. A good rule of thumb is always dose buffers in the AM when pH is at it's lowest. It will impact the tank much less. The Kent can have a decent affect on pH as well. How soon after you can dose the calcium (part B) will depend on the amount of water flow you have but generally speaking an hour or so should be fine. If you are not sure wait longer.
Cheers
Steve