Purity: Multiple Obligatory Ghusls
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 387:
Someone who must perform a number of obligatory ghusls can perform one ghusl with the intention of all of them. Similarly, if he makes the intention of one of the ghusls, it is sufficient for the others [and he does not have to make separate intentions].
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)