Thihathu and Sawyun[ပြင်ဆင်ရန်]
Pardon me for writing in English. It's much quicker for me. (Not very good with the Burmese keyboard.)
It wasn't Thihathu's decision to allow Sawyun to be the rival king. We *know* that Thihathu never tolerated any rivals. In 1310, he killed his own brother to become the sole king! Even when his brothers were alive, he was declaring himself as the king. As early as 1295 (when the brothers were viceroys under Kyawswa of Pagan), Thihathu was claiming himself to be *the* king. It was because of Thihathu that Kyawswa got nervous, and sought Mongol help in 1297.
Sawyun left for Sagaing because he was unhappy with Thihathu's decision to appoint Uzana as heir apparent. Uzana (son of Kyawswa of Pagan) was Thihathu's adopted son. Sawyun felt, he as the eldest biological son, should have been the heir apparent. Although Sawyun *nominally* remained loyal to Thihathu at Sagaing, his departure to Sagaing was a thinly-veiled rebellion. Thihathu, perhaps because of his old age and/or guilt, tolerated Sawyun. When Sawyun died in 1323 (at age 22), his successor Tarabyagyi simply ignored Thihathu and proceeded to make himself king. The point is Thihathu didn't voluntarily allow the founding of Sagaing Kingdom.
Hope it helps.Hybernator ၁၆:၅၆၊ ၂၄ အောက်တိုဘာ ၂၀၁၀ (UTC)