Is it permissible to say to someone: "You are in the safety of God and my safety," without connecting it with "then" (ثم), and is it better to use "then" (ثم) out of reverence for God?

The correct phrasing is: "You are in Allah's protection, then in my protection." It is stated in Sheikh Bakr Abu Zayd's book, "Al-Manahi al-Lafziyyah" (Prohibited Phrases): If one says, "I am by Allah, then by you," or "What Allah willed, then what you willed," there is no harm in that. He supports this by citing the hadith: "I have no report today except by Allah, then by you," and the hadith that permits saying: "What Allah willed, then what so-and-so willed." In Al-Baghawi's "Sharh al-Sunnah" (Commentary on the Sunnah): "Do not say, 'What Allah willed and what so-and-so willed,' but say, 'What Allah willed, then what so-and-so willed.'"