The first of the two Mandarin characters for Tibet is "West". What is the second? Thanks.

²Ø (z¨¤ng): hide, conceal, store.
"Hidden Treasure In The West" ...or at least the romantic in me remembers it that way.

"Zang" is the word for the Tibetan nationality. The word "West" is used because it refers to the western region of the Tibetan nationality (which also extends east into Sichuan, Qinghai, and parts of Gansu and Yunnan).
Some Tibetans may be offended by translating their name as "Treasures in the West".
The word "zang" is derived from U-Tsang during the Qing Dynasty. U-Tsang was one of the major states in Tibet, which included most of the TAR nowadays. Tibetan people call themselves "bod" (and thus tiBET), Chinese call them only "zang" because Chinese generalize the whole area of Tibet as "zang".
Technically Amdo or Kham people should be "bod" but not "zang", just like Scots are Brits but not English, but nobody seems to care.

"Treasures in the West" is a literal translation adapted to the name Xizang. It didn't give rise to the name Xizang.
#3,
"Zang" is a phonetic transliteration of the historic region of Tsang, not U-Tsang. It then came to mean "Tibet" in Chinese and replaced Tufan which had been the previous Chinese word for Tibet. So Amdo and Kham can also be described in Chinese as "Zang". You can't call them "Bod" in Chinese anyway because no such transliteration of the word is in accepted use.