What is the term for two file formats that describe the same amount of information?
I think that many programmers would look at it in terms of the conversions between the two formats, and say that each format can be converted to the other losslessly, and that it's possible to make a round-trip without losing any data. It's not a standard usage, but if you said that the two formats were round-trippable wrt each other, you would probably be understood. That said, you should still explain what that means!