あと vs もう as taught in JFZ 1 and 2
In JFZ1, he teaches us to use あと when asking for more of something: ピザをあとにまい、ください。
Then in book two, when we get to the uses of もう, one of them is to ask for more of something: ピザをもうにまい、ください。
He doesn’t mention that あと was taught earlier for “more” or explain the difference. They seem identical. So my question is, is there a difference in usage or is one more commonly used when asking for more of something?