Both can mean "all at once," so are they interchangeable or is there a different nuance to each?
一気に is like saying “in one breath” and it always describes the quickness of some action, while 一度に is more like “at a time” in the sense of "per time" or “not on separate occasions”, and it says nothing about the duration. It may just happen to be a short time.
He drank three glasses of water in one go.
He drinks three glasses of water at a time (= per time).