Is it permissible for me to intend to pray the Shafa' (even-numbered) prayer behind an Imam who intends to pray the the ninth and tenth rak'ahs of the Tarawih prayer?

It is permissible for the intention of the follower to differ from the intention of the Imam in all cases, as long as the apparent actions of the two prayers do not differ. This is the madhhab of Imam Shafi'i and those who agreed with him.