"Are you going to the cinema?"
Translation:Dych chi'n mynd i'r sinema?
dych chi is the second person plural form, so you would use it when speaking to several people, or when speaking politely to one person.
wyt ti is the second person singular form, so you would use it when speaking to one person whom you know well (roughly, that you're on first-name terms with).
So they're not interchangeable.