you can use them both together. probably be better off splitting up the buffer dose into several doses ( ex if your dosing 12ml on water change day just do 2ml a day or 4ml every other day). I wouldn't dose on water change day myself personally. this is the one day of the week you are already adding plenty of carbonate and bicarbonate via salt mix. dont dose calcium and buffer at the same time. dose the buffer in the am and calcium in the pm or on opposite days.
BTW there is nothing special about using the same brand part a and part b other than the ca part is watered accordingly so you can dose a and b in equal doses. Stand alone calcium chloride products cost the same and are much more concentrated/economical. I test and have no desire to dose ca or alk equally based on test results of the other. I dose 3x the buffer I do calcium. If your plan on using Pro Buffer dKH in the future dont replace the b-ionic with b-ionic or any 2 part calcium. Kent liquid calcium is nearly twice as concentrated as B-ionic 2 part calcium ( or any 2 part calcium. I checked with a trusted reef calculator). Use kent liquid calcium, seachem reef complete, brightwell cacion, blue life precision calcium ect (and kent liquid calcium is the least concentrated of those and still whops any 2 part). If you want to dose a 2 part look into BRS.
