No, they won't react with each other. I assume you are saying that you are measuring out the mashing and sparging additions based on their water volumes the night before and just leaving them on the plate, premeasured. That is OK.
But you wouldn't want to store some of your minerals outside of a closed container since they can pick up moisture. This is particularly important for calcium chloride, since it will suck moisture from the air and you will end up with a wet mess if that stuff is left out long enough.