at equilibrium the ion concentration for both is 4.99x10-3 M
Not necessarily.
how would they both get to the same concentration??
They won't. Equilibrium doesn't mean "same concentrations". It means "no further changes in concentrations".
Yes, for reactions like dissolution of CaSO
4 (or AgCl) you can expect concentrations of both ions to be the same, but that's just because of the reaction stoichiometry. What about dissolution of Ag
2S? Ca
3(PO
4)
2?
What will happen in your case is that some CaSO
4 will precipitate till concentrations of whatever is left will be described by the solubility product (Ksp) of the compound.