A solution is made of solute and solvent, like salt in water, however, what if the solute and solvent exist in equal quantities? Which one is the solute and which is the solvent? If one of them is water I think it will definitely be the solvent, but what if neither of the two parts are water?
As far as I can recollect, I think in such a situation you can arbitrarily select one of them as solute and one as solvent. However, according to convention, it is better to assign the solvent as a substance which is usually used as a solvent in other situations.