Moldavia and Wallachia certainly properly describes "Moldavia and Wallachia" ... . :)
It makes no sense to introduce a retrospective political term for 3 categories only, which would describe themselves in themselves, i.e 1)Moldavia 2) Wallachia 3) Moldavia &Wallachia.
The term "romanian" applied retrospectively in history would be a major historic buff. and hence no way to go!