Phase diagram for Na2B4O7 and NaCI mixture

A. Most mixtures of oxides (or salts of oxygenated anions) and halides display a wide immiscibility gap in the molten state. There are some exceptions, typically (I think) for cations for which the free energy of formation per mole of M-O "bonds" is not very far from the free energy of formation per mole of M-Cl "bonds" (using quotation marks because I am talking about ionic crystalline materials), but this is definitely not the case for Na.

So I would say your observation is correct, and people probably work with a two-phase liquid molten salt. In fact I have a vague recollection of seeing some data on immiscibility for this particular system, but wasn't able to locate it. The viscosities and conductivities of the two phases are surely rather different, but as long as the system is mixed well, and in a measurably reproducible way, you should be in good shape.