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. (See: [[rc://en/ta/man/translate/translate-ordinal]])
Here the phrase "the whole law and the prophets" refers 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]])