Well, first of all I have to say that "Ты можешь говорить по-русски?" and "Можете ли вы говорить по-русски?" are really the same thing most of the time. The only difference is that вы can be used as plural you (like all of you, two of you etc) or a polite singular you. So if you don't really know a person it's better to use the variant with вы because it is polite, however if you're friends with that person you can safely use the ты variant. Anyway if you're adressing more than one person with that question you definitely use вы and nothing else.

Secondly, the question as you wrote it doesn't sound natural though understandable. In real world it would be either "ты говоришь по-русски?" or "вы говорите по-русски?".