This is a metaphor to illustrate their close union as husband and wife. AT: "the two people are like one person" or "they are no longer two, but together they are one body" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The phrase "what God has joined together" refers to any married couple. AT: "Therefore since God has joined together husband and wife, let no one tear them apart" (See: [[rc://en/ta/man/translate/figs-explicit]])