It's similar whether it's the anion or cation which is more than one. The enthalpy changes that involve the metal cation are the enthalpy change of atomisation of it and ionisation energies of it. So, you just have to multiply those values accordingly with the number of metal cations in the compound.
For example, comparing between NaCl and Na2O..
For NaCl, it's just Na (s) -> Na (g) for the atomisation part.
For Na2O, (since there are 2 Na), it'll be 2 Na (s) -> 2 Na (g). Hence, you multiply the enthalpy change of atomisation of Na by 2.
Hope this helped.
Edit: oops, didn't see excalibur post before me..haha