Complex sentence: Flatt had also made up his mind to leave but he had not told anyone but later gave his two-week notice; before the notice was up, the bass player Howard Watts announced that he was leaving too.
Split into simpler sentences: Flatt had also made up his mind to leave but he had not told anyone. He later gave his two-week notice but, before the notice was up, the bass player Howard Watts announced that he was leaving too.