Why does Zouken need both Illya and Sakura?


During the HF route, Zouken wants to use both Illya and Sakura as Grails. His
plan is to summon the Greater Grail to give birth to Avenger. But in Sakura
there are the worms of Zouken. When Avenger assimilates Sakura, Zouken will
take possession of his body through his worms.

Illya and Sakura are Lesser Grails with the same functions, so why does Zouken
need both? In fact, one is enough. After Shirou and Kotomine saved Illya,
Zouken proceeded to summon the Greater Grail using only Sakura.

