A typical anticorrosion layer is thin, like 10 to 100µm, so its deposition alters very little the composition of the bath, and the current and the mass losses at the counter-electrodes won't represent the deposited composition neither. I can imagine only to analyse the deposited layer.
Is Mg useful in an anticorrosion layer? Up to now, I've seen a protection only by elements that are themselves resistant to corrosion, like Cr, Al, Zn, Ni and others, but not Mg. I also suspect Mg is badly difficult to deposit electrochemically, especially from an aqueous solution.
And what do you call Tn? Is it Sn?
You might also want to decide if the metal layer shall resist corrosion by its redox potential, like Au does, or by its oxide layer, like Cr does. I haven't seen a deposited layer containing both up to now.
An other limit to electrolytic co-deposition is that the redox potentials of the elements (that's written too shortly) must not differ too much. Co and Ni yes, Cu and Sn yes, but Zn and Sn I doubt.