The need for two distinct classifiers can be illustrated with the following example: ( (1a,b) are grammatical while (1c) is hardly acceptable, and no simple precedence rule can be learned from pairs of constituents in (1a) and (1b): the temporal adjunct earlier today can precede or follow each of the other constituents dependent on the verb (she, him, an email).
the classifier which determines the precedence relation is not enough.