What is the same time? Good question I doubt there is a definitive answer.
Just out of interest back when I used a 2 part additive I switched to dosing them with a 2 channel dosing pump, the one pump dosed both at the same time, but they dripped in very slowly, alk to one end of the tank, and ca to the other. I positioned them so they dripped into high flow, and ( in theory ) would be well mixed by the time they met. However the amount I had to use increased heaps, it must have been precipitation. I eventually abondoned this method.
On RC they often recommend dosing one in the morning, one in the evening. I've tried 1/2 hour apart and this has seemed to work. Layton could probably explain better if he's around.