Complex sentence: Revere and Dawes prepared to ride toward Concord, where the militia's arsenal was hidden; they were joined by Samuel Prescott, a doctor who lived in Concord and happened to be in Lexington.
Split into simpler sentences: Revere and Dawes then rode toward Concord, where the militia's arsenal was hidden. They were joined by Samuel Prescott, a doctor who happened to be in Lexington.