践 (to tread/perform) and 祚 (throne/reign) both use their On'yomi readings to form せんそ. 祚は read ソ, not サ, and does not undergo rendaku to ゾ. This term signifies the formal act of an emperor ascending to the throne.

践も祚も音読みで「せんそ」と読みます。祚は「さ」や「ぞ」とは読みません。天皇が皇位を継承し、その位に就くことを指す厳粛な儀式や行為です。