Is a subsequent designation of an executor valid if a prior designation exists?

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If a testator states, 'I appointed Zayd as my executor,' and then says, 'If Zayd dies, my appointment is for 'Amr,' this is valid according to a single narration. Both Zayd and 'Amr become executors, but 'Amr acts only after Zayd. This is supported by analogy with the Prophet's designation of commanders for the Battle of Mu'tah: 'Your commander is Zayd; if he is killed, your commander is Ja'far; if he is killed, your commander is 'Abdullah ibn Rawahah.' The principle of bequest (wasiyyah) is analogous to the principle of designation (ta'meer).