Unable to setup mapping set with two category code as input source

Summary: We have requirement to map the cost center based on the combination of product class and subclass value. The product class and subclass are setup as two catalogs.

E.g. Product class : ABC and Subclass : X then Cost center : 001

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We have requirement to map the cost center based on the combination of product class and subclass value. The product class and subclass are setup as two catalogs.

E.g. Product class : ABC and Subclass : X then Cost center : 001

We are unable to setup mapping set for the combination. We assigned Product class catalog to Costing functional area.