Both of calcium's electrons are in its innermost shell. Why is this?
Calcium has 20 electrons, not 2 as implied by the word, "both".
The atomic number of calcium is 20 which means it has 20 protons and as an atom (not an ion) it also has 20 electrons. These are in four energy levels (1 though 4) and it has the following electron configuration.
The first two (