The balanced chemical equation for reaction between calcium and magnesium ions and EDTA is:

$Ca_2 +H_2EDTA^{2-}\rightarrow CaEDTA^{2-}+2H^+\\[2ex] Mg_2+H_2EDTA^{2-}\rightarrow MgEDTA^{2-}+2H^+$

So if we let $M^{2+}$ be the total of $Ca^{2+}$ and $Mg^{2+}$ ions in the solution then:

$M^{2+}+H_2EDTA^{2-}\rightarrow MEDTA^{2-}+2H^+$

1 mole of $M^{2+}$ reacts with 1 mole of $H_2EDTA^{2-}$.

At the equivalence point of titration:

$\text{moles}M^{2+}=(\text{moles}Ca^{2+}+\text{moles}Mg^{2+})=\text{moles}H_2EDTA^{2-}$