As long as one is mixed in, you can add the second. What you don't want is two concentrated clouds of the chemical to interact. That can lead to precipitation of calcium carbonate.
So a half hour is almost always enough. In some tanks, just one minute may be plenty if there is lots of flow.
Thanks for this info,i wasent really sure i just did them 3 hours apart lol.
i do alk at night and cal during the day. is that okay?