K+ vs Mg+ Replacement
I have usually replaced magnesium first then the Potassium for the reason that Magnesium activates adenosine triphosphatase (ATPase), which is essential for normal cell membrane function and is the energy source for Na-K pump and intracellular magnesium deficiency may cause an increase in intracellular Na and Ca and a loss of K.

But then again, I am always tempted to replace K first when it has a low critical value. In a case of K+=3.0 and a Mg+=1.8, what would you replace first?