The two sides (front and back) are one and inseparable; used metaphorically as 'two sides of the same coin'.
利益と責任は表裏一体であり、得るだけでは済まない。
Benefits and responsibilities are two sides of the same coin; you can't just take the benefits.
自由と秩序は表裏一体の関係にあると考えられている。
Freedom and order are thought to be inseparably related.
彼の優しさには厳しさが伴い、表裏一体の性格だ。
His kindness comes with strictness; he has a two-sided but unified character.