As you cast a modal double-faced card with Kari Zev's Expertise, you choose which face (not: half) to cast, and "[o]nly the face that will be face up on the stack is evaluated to determine if it can be cast" (C.R. 711.7, 711.7a). For this reason, you can cast a face of a modal double-faced card with Kari Zev's Expertise only if that face's converted mana cost is 2 or less (as is the case for the Valki face, but not for the Tibalt face).
Note that Kari Zev's Expertise now says "You may cast a spell ... from your hand..." (C.R. 108.1); it no longer says "You may cast a card ... from your hand...".
As you cast a modal double-faced card with Kari Zev's Expertise, you choose which face (not: half) to cast, and "[o]nly the face that will be face up on the stack is evaluated to determine if it can be cast" (C.R. 711.7, 711.7a). For this reason, you can cast a face of a modal double-faced card with Kari Zev's Expertise only if that face's converted mana cost is 2 or less (as is the case for the Valki face, but not for the Tibalt face).
Note that Kari Zev's Expertise now says "You may cast a spell ... from your hand..." (C.R. 108.1); it no longer says "You may cast a card ... from your hand...".
See also this thread and this thread.