Most common use and difference is that the second form "kako si ti?" you will use as a question after someone has already asked "you kako si?" example: Kako si? dobro, hvala. Kako si ti? you will never start a conversation with "kako si ti?" unless you are trying to make an emphasis to who are you talking to, for example, if there are two persons in front of you and you want to ask only one of them. And " kako si ti" usually precedes another verb like: Kako si ti uspeo da.... How did you manage to.... I hope this was helpful!

There is no difference between those two phrases, but the first phrase is more common. And "(a) kako си ти", can be the answer/reaction to the question "Kako си?". The situation : Здраво, како си? Хвала добро (а) како си ти ? N.B. The pronoun ТИ in the case of a text, can be written with the capital letter as a sign of the rescpect. (Ex.: Драга бако Ти ме увек изненадиш и Тебе се увек присетим ... Good luck !

When we use CИ, we already have grammatical information, that it is about second person singular - YOU: Како си? - How are YOU ? Где си? - Where are YOU ? Одакле си? - Where are YOU from? Да ли си иморна? - Are YOU tired ? This is the reason, why it isn't necessary to use ТИ in sentences, where we use "си".

Како си ти can be used as answer when someone asks you firstly Како си. You never start conversation with Како си ти, Како си is enough