Hi Susan,
it is recommended you pick only one of the products. They have different concentrations and mixing multiples, would require a more complex calculation to account for the difference in concentration, when determining the daily dosage.
Seachem has at least 3 calcium supplement products, as far as I can see and they have different concentrations of Ca. Also, Reef Calcium is Ca only and the other two (Reef Advantage Ca and Reef Complete) are Ca + Mg + Sr.
You would choose the concentration depending on price and how you dose. Usually, higher concentration is cheaper.
Also, you would pick Mg + Sr if you wish to replenish these elements without having to dose them separately. My advice in this case is to not dose unless you can test these elements.