What is the primary basis for prohibiting individual action when joint appointment is made without specification?

Primary text

The primary basis for prohibiting individual action when the appointment is joint and unspecified is that the testator shared the authority between them, meaning neither possesses the authority to act alone, analogous to two agents (wakilayn). The position of Abu Yusuf, allowing individual action, is rebutted by noting that if the testator intended the authority to be joint, it is not divisible, just as if he had explicitly stated they must only act jointly, or if he appointed two agents.