Complex sentence: It is believed that Francesco and Bianca were given small doses of arsenic for several days until it killed them, but the doses were probably too small and given over a too short period of time to be detected in Francesco's facial hair.
Split into simpler sentences: It is believed that Francesco and Bianca were given small doses of arsenic for several days until it killed them. But the doses were probably too small and given over a too short period of time to be detected in Francesco's facial hair.