这个句子涉及并列句的省略和名词性替代。分号相当于并列连词。

Morocco’s king bears the title Commander of the Faithful; Saudi Arabia’s that of Custodian of the Two Holy Mosques.

= Morocco’s king bears the title Commander of the Faithful; Saudi Arabia’s king bears the title Commander of Custodian of the Two Holy Mosques.

蓝色部分为省略，红色部分用that替代。