How to read and write to a file via two file descriptors (without using pointer movement functions)

Task: let there be two file descriptors (stream variables, file variables) in and out. It is required to write out several numbers through out, and then, without moving the pointer (for example, using seekg or seekp, since these functions are unacceptable in time), through in read the recorded numbers. The problem is that in c ++, as I understand it, even if two file variables are created that are associated with one file, the internal pointer in the file that moves through the positions in the file is still one. E