Possible meanings are 1) "there is another commandment that is next in importance" or 2) "there is a second commandment that is just as important." Either way, Jesus means these two commandments are more important than all other commandments.
Here "whole law and the prophets" refer to all of scripture. AT: "Everything that Moses and the prophets wrote in the scriptures is based on these two commandments" (See: [[rc://en/ta/man/translate/figs-metonymy]])