In calligraphy, if two words are repeated, can the second word be replaced by two points?

Of course. This usage is very common in ancient and modern calligraphy works.

There is a saying in Zhao Mengfu's lower-case Tao Te Ching in the Yuan Dynasty: "Many things return to their roots."

There is a saying in Mi Fei's Tiaoxi Poems in the Northern Song Dynasty: "Only pity the bright moon is still full."