Uddin means “of the faith” (i.e. “of Islam”), thus Saifuddin means “sword of the faith” (sword of Islam). Saifuddin is made up of two separate words and in Arabic they are actually written with a space between them (سيف الدين). There is no issue with writing it as Saif Uddin. Some people write it as three separate units: Saif Ud Din, this too is acceptable.