A reaction involving two different reactants can never be:

Molecularity is defined as the number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction which must collide simultaneously in order to bring about a chemical reaction. If a reaction involves two different reactants ($A+B\to \text{Product}$), at least two molecules must collide, so the molecularity must be at least $2$. It can never be unimolecular (molecularity $=1$).