Question

how to calculate the concentration of mixed solutions like NaNO3 + Ca(CO3)2 ?

Answer 1

Concetration of what? The concentration of any one component, e.g. the NaNO3, is unaffected by the presence of any other component, and you calculate it as if no other component was present.

You might mean total particle concentration, taking no account of the identity of the particles, which is what you would use if you were considering colligative properties (because these don't depend on the identity of the particles). In that case, you would simply add all the moles of Na+, NO3-, Ca-2 and CO3-2 together, then divide by the volume of the solution to get the molarity, or divide by the mass of the solvent to get the molality.