both have the same meaning. Particle "kah" It serves as an affirmation in questions. Whether the particles are used to replace the word question "whether", and the special interrogative sentence.   Particle "kah" has functions as emphasizing the sentence in question. It is used toreplace question word "if" (yes-no question), and can occur in specific question. 1. to replace the interrogative sentence (whether = "if") To form a question (whether = "if"). Particle-kah can be attached to a noun (subject and object) also to verbs, linking verb and adjective (predicate). Example: Diakah temanmu? (Apakah dia temanmu?) = Is he your friend? Datangkah dia? (Apakah dia datang ?) = Did she come? Sulitkah belajar bahasa Indonesia? (Apakah belajar bahasa Indonesia sulit?) = Is it difficult to learn Indonesian? 2. asked to affirm the sentence by adding words whether behind the question. It occurs in specific question. Toemphasize a question by adding -kah at the end of a question words Example: Berapakah saya harus membayar dia? (Berapa saya harus membayar dia?) = How much I have to pay him? Siapakah orang yang berbaju putih itu? (Siapa orang yang berbaju putih itu?) = Who is wearing the white shirt? Hope it helped!

I think "Siapa" same with "Siapakah" looked from meaning. Siapa usualy use in formal sentence .. Siapakah usualy use in speaking .. -kah usualy use in speaking.. but sometimes -kah also use in sentence (new/ magz) Siapa is more formal than siapakah