A reaction involving two different reactants can never be (a) unimolecular reaction

(a) : Generally, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation. Thus, a reaction involving two different reactants can never be unimolecular. 

But a reaction involving two different reactants can a first order reaction. For example, for the following reaction 

RCl + H₂O → ROH + HCl Expected rate law : 

Rate = k[RCl][H₂O] expected order = 1 + 1 = 2 

But actual rate law : Rate = k'[RCl] actual order = 1 

Here water is taken in excess, hence its concentration may be taken constant. 

Here the molecularity of the reaction = 2 and the order of the reaction = 1.