TT #228 · Parshas Tazria · Aliyah 7 · Passuk 42
English: But if a white-reddish lesion appears on the bald area or the front bald area, it is an eruptive tzaraas on his bald area or front bald area.
Torah Temimah cites Sifra (Toras Kohanim) for the rule that karachas and gabachas do not combine.
English: The verse teaches that a lesion on the karachas area and one on the gabachas area are not combined.
The two baldness regions remain separate for the law.
The posterior bald area (karachas) and frontal bald area (gabachas) are distinct regions; a lesion in one region cannot be added to a lesion in the other to reach the required measure.