Why do they say that one must not visit both Sri Kalahasti and Tirupathi?

There is no widespread or traditional prohibition against visiting both Sri Kalahasti and Tirupati temples. In fact, many devotees plan their visits to include both temples, as they are relatively close to each other, with Sri Kalahasti being about 36 kilometers from Tirupati.

The temples are often visited together as part of a pilgrimage, with devotees performing rituals and seeking blessings at both sacred locations5.

Therefore, there is no specific reason or tradition that suggests one must not visit both Sri Kalahasti and Tirupati.