TT #229 · 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 a lesion does not spread from karachas into gabachas or vice versa.
English: One might think that, although a lesion in karachas and a lesion in gabachas do not combine to meet the required measure, a lesion could nevertheless spread from one area into the other. The Sifra excludes that too: spread from one area does not count in the other.
The separation applies to spreading as well as measurement.
A lesion cannot establish spreading by extending from karachas into gabachas or vice versa.