coincidental is more common than coincident. A LOT more common. Don't worry about using coincident in everyday conversation. Btw, the noun form (coincidence) is most common. "What a coincidence!" means 真巧.

It is a coincidence that we are here at the same time. It was a coincidental meeting. This means it was by chance. Nothing causes a coincidence. It just happens. You would not need to use the word coincident. coincidence is a noun. coincidental _______ is an adjective.